diff --git a/src/bin/not.rs b/src/bin/not.rs
index 81d21ad..a538966 100644
--- a/src/bin/not.rs
+++ b/src/bin/not.rs
@@ -1,4 +1,4 @@
-use not_python::{syn::ast, compile::Compile, vm::{Vm, signal::*}};
+use not_python::{syn::ast, compile::Compile, vm::Vm};
 
 use std::{fs, path::PathBuf};
 use structopt::StructOpt;
@@ -57,8 +57,7 @@ fn main() -> Result<()> {
 
     if action == "run" {
         let mut vm = Vm::new(&const_pool);
-        vm.handle_signal(Signal::Call(main, vec![]));
-        vm.resume();
+        vm.call(main, vec![])?;
     }
 
     Ok(())
diff --git a/src/compile/mod.rs b/src/compile/mod.rs
index f5122b6..84b4a19 100644
--- a/src/compile/mod.rs
+++ b/src/compile/mod.rs
@@ -42,7 +42,7 @@ impl Compile {
                 sym
             })
             .collect();
-        Ok((self.const_data.const_pool, UserFun::new_obj(main, globals)))
+        Ok((self.const_data.const_pool, UserFun::new_obj(main, globals, 0)))
     }
 
     /// Gets the constant data that is interned in this compile session.
diff --git a/src/compile/thunk.rs b/src/compile/thunk.rs
index ad4c198..47b571b 100644
--- a/src/compile/thunk.rs
+++ b/src/compile/thunk.rs
@@ -407,7 +407,7 @@ impl Visit for CompileBody<'_> {
             })
             .collect();
 
-        let (hdl, _fun) = self.compile.push_const(UserFun::new_obj(code, locals));
+        let (hdl, _fun) = self.compile.push_const(UserFun::new_obj(code, locals, expr.params.len()));
 
         // TODO(compile) : determine return value at the end of the body (preferably at parse-time)
 
diff --git a/src/obj/builtin.rs b/src/obj/builtin.rs
index 45852e9..c9fd651 100644
--- a/src/obj/builtin.rs
+++ b/src/obj/builtin.rs
@@ -1,45 +1,46 @@
-use crate::{obj::{prelude::*, reserved::*}, vm::signal::*};
+use crate::{obj::{prelude::*, reserved::*}, vm::{error::*, signal::*}};
 use maplit::btreemap;
 use once_cell::sync::Lazy;
 use std::collections::BTreeMap;
 
-pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(|_, vm, args| {
+pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(1, |_, vm, args| {
     // TODO : use __get_attr__ when it gets added
     let to_string = {
         let obj_ref = &args[0];
         read_obj!(let obj = obj_ref);
         obj.get_attr(STR_MEMBER_NAME.sym)
-            .or_else(|| obj.get_attr(REPR_MEMBER_NAME.sym))
-            .expect("no __str__ or __repr__ member")
+            .ok_or(Error::MissingAttr { attr: STR_MEMBER_NAME.sym })?
     };
-    let return_value = vm.call(to_string, vec![]);
+    let return_value = vm.call(to_string, vec![])?;
     {
         read_obj!(let str_obj = return_value);
-        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
+        let str_obj: &Str = str_obj.as_any().downcast_ref()
+            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), value: return_value.clone() })?; 
         println!("{}", str_obj.value());
     }
 
     vm.push(NIL_NAME.sym_ref());
-    Signal::Return
+    Ok(Signal::Return)
 }));
 
-pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(|_, vm, args| {
+pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_obj(1, |_, vm, args| {
     // TODO : use __get_attr__ when it gets added
     let to_string = {
         let obj_ref = &args[0];
         read_obj!(let obj = obj_ref);
         obj.get_attr(STR_MEMBER_NAME.sym)
-            .or_else(|| obj.get_attr(REPR_MEMBER_NAME.sym))
-            .expect("no __str__ or __repr__ member")
+            .ok_or(Error::MissingAttr { attr: STR_MEMBER_NAME.sym })?
     };
-    let return_value = vm.call(to_string, vec![]);
+    let return_value = vm.call(to_string, vec![])?;
     {
         read_obj!(let str_obj = return_value);
-        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
+        let str_obj: &Str = str_obj.as_any().downcast_ref()
+            .ok_or_else(|| Error::ValueError { error: "expected str value".to_string(), value: return_value.clone() })?;
         print!("{}", str_obj.value());
     }
 
-    Signal::Return
+    vm.push(NIL_NAME.sym_ref());
+    Ok(Signal::Return)
 }));
 
 pub static BUILTIN_OBJS: Lazy<BTreeMap<Sym, ObjRef>> = Lazy::new(|| btreemap! {
diff --git a/src/obj/fun.rs b/src/obj/fun.rs
index 2c365d4..7bd7bbf 100644
--- a/src/obj/fun.rs
+++ b/src/obj/fun.rs
@@ -1,4 +1,4 @@
-use crate::{obj::{reserved::*, prelude::*}, vm::{consts::ConstPool, frame::*, inst::Inst, signal::*, Vm}};
+use crate::{obj::{reserved::*, prelude::*}, vm::{consts::ConstPool, error::*, frame::*, inst::Inst, signal::*, Vm}};
 use once_cell::sync::Lazy;
 use shredder::{GcSafeWrapper, Scan};
 use std::{fmt::{Debug, Formatter, self}, io::{self, Write}, sync::Arc};
@@ -8,7 +8,7 @@ pub type FunLocals = Vec<Sym>;
 /// A function object which can create a new stack frame.
 pub trait Fun {
     /// Creates a new `vm::Frame` from this function object.
-    fn create_frame(&self, callee: ObjRef, vm: &Vm, _args: Vec<ObjRef>) -> Frame;
+    fn create_frame(&self, callee: ObjRef, vm: &Vm, _args: Vec<ObjRef>) -> Result<Frame>;
 }
 
 pub type MethodRef = ObjRef<Method>;
@@ -55,7 +55,7 @@ impl Obj for Method {
 }
 
 impl Fun for Method {
-    fn create_frame(&self, _callee: ObjRef, vm: &Vm, mut args: Vec<ObjRef>) -> Frame {
+    fn create_frame(&self, _callee: ObjRef, vm: &Vm, mut args: Vec<ObjRef>) -> Result<Frame> {
         // insert self argument
         args.insert(0, self.get_attr(SELF_MEMBER_NAME.sym).unwrap());
         // get function and use its create_frame method with the updated args
@@ -82,16 +82,18 @@ pub struct UserFun {
     // Safe because this is just an interner that points to symbols, which aren't GC'd
     #[shredder(unsafe_skip)]
     locals: FunLocals,
+    arity: usize,
 }
 
 impl UserFun {
-    pub fn new_obj(code: Vec<Inst>, locals: FunLocals) -> UserFunRef {
+    pub fn new_obj(code: Vec<Inst>, locals: FunLocals, arity: usize) -> UserFunRef {
         self_referring_obj! (
             Self {
                 vtable: Default::default(), // this is a placeholder for the real vtable
                 attrs: Default::default(),
                 code: Arc::new(code),
                 locals,
+                arity,
             },
             vtable: |obj_ref: ObjRef| vtable! {
                 TY_MEMBER_NAME.sym => USER_FUN_TY.clone(),
@@ -215,13 +217,19 @@ impl Obj for UserFun {
 }
 
 impl Fun for UserFun {
-    fn create_frame(&self, callee: ObjRef, vm: &Vm, args: Vec<ObjRef>) -> Frame {
+    fn create_frame(&self, callee: ObjRef, vm: &Vm, args: Vec<ObjRef>) -> Result<Frame> {
+        if args.len() != self.arity {
+            return Err(Error::ArityError {
+                expected: self.arity,
+                got: args.len(),
+            });
+        }
         let bindings: FrameBindings = args.into_iter()
             .enumerate()
             .collect();
-        Frame::User(
+        Ok(Frame::User(
             UserFrame::new(callee, bindings, vm.pc(), Arc::clone(&self.code))
-        )
+        ))
     }
 }
 
@@ -241,7 +249,7 @@ pub static USER_FUN_TY: Lazy<ObjRef<Ty>> = Lazy::new(|| Ty::new_obj(USER_FUN_NAM
 // struct NativeFun
 //
 
-pub type NativeFunPtr = fn(ObjRef, &mut Vm, Vec<ObjRef>) -> Signal;
+pub type NativeFunPtr = fn(ObjRef, &mut Vm, Vec<ObjRef>) -> Result<Signal>;
 pub type NativeFunRef = ObjRef<NativeFun>;
 
 #[derive(Scan)]
@@ -250,6 +258,7 @@ pub struct NativeFun {
     attrs: Attrs,
     #[shredder(skip)]
     fun: GcSafeWrapper<NativeFunPtr>,
+    arity: usize,
 }
 
 //
@@ -257,12 +266,13 @@ pub struct NativeFun {
 //
 
 impl NativeFun {
-    pub fn new_obj(fun: NativeFunPtr) -> ObjRef<Self> {
+    pub fn new_obj(arity: usize, fun: NativeFunPtr) -> ObjRef<Self> {
         self_referring_obj! (
             Self {
                 vtable: Default::default(),
                 attrs: Default::default(),
                 fun: GcSafeWrapper::new(fun),
+                arity,
             },
             vtable: |obj_ref: ObjRef| vtable! {
                 TY_MEMBER_NAME.sym => NATIVE_FUN_TY.clone(),
@@ -313,10 +323,16 @@ impl Obj for NativeFun {
 }
 
 impl Fun for NativeFun {
-    fn create_frame(&self, callee: ObjRef, _vm: &Vm, args: Vec<ObjRef>) -> Frame {
-        Frame::Native(
+    fn create_frame(&self, callee: ObjRef, _vm: &Vm, args: Vec<ObjRef>) -> Result<Frame> {
+        if args.len() != self.arity {
+            return Err(Error::ArityError {
+                expected: self.arity,
+                got: args.len(),
+            });
+        }
+        Ok(Frame::Native(
             NativeFrame::new(callee, *self.fun, args)
-        )
+        ))
     }
 }
 
diff --git a/src/obj/int.rs b/src/obj/int.rs
index e1b47a3..405e1c8 100644
--- a/src/obj/int.rs
+++ b/src/obj/int.rs
@@ -44,13 +44,14 @@ impl Debug for Int {
 }
 
 static INT_STR_FUN: Lazy<NativeFunRef> = Lazy::new(|| {
-    NativeFun::new_obj(|_callee, vm, args| {
+    NativeFun::new_obj(1, |_callee, vm, args| {
         read_obj!(let int_obj = &args[0]);
         if let Some(int_obj) = std::any::Any::downcast_ref::<Int>(int_obj.as_any()) {
             vm.push(Str::new_obj(int_obj.value.to_string()));
         } else {
             panic!("{:?} is not an int", int_obj)
         }
-        crate::vm::signal::Signal::Return
+
+        Ok(crate::vm::signal::Signal::Return)
     })
 });
diff --git a/src/obj/mod.rs b/src/obj/mod.rs
index 737d83f..70f6307 100644
--- a/src/obj/mod.rs
+++ b/src/obj/mod.rs
@@ -20,6 +20,7 @@ pub mod prelude {
 
 use shredder::{Gc, Scan};
 use std::{
+    fmt::{self, Debug, Formatter},
     marker::Unsize,
     ops::{CoerceUnsized, Deref, DerefMut},
     sync::RwLock,
@@ -42,7 +43,7 @@ macro_rules! obj_attr {
     }
 }
 
-pub trait Obj: Scan + std::fmt::Debug {
+pub trait Obj: Scan + Debug {
     fn vtable(&self) -> &Vtable;
     fn attrs(&self) -> &Attrs;
     fn attrs_mut(&mut self) -> Option<&mut Attrs>;
@@ -82,7 +83,7 @@ pub trait Obj: Scan + std::fmt::Debug {
 // struct ObjRef
 //
 
-#[derive(Debug, Scan)]
+#[derive(Scan)]
 pub struct ObjRef<T = (dyn Obj + Send + Sync + 'static)>
 where
     T: Obj + ?Sized + Send + Sync,
@@ -135,6 +136,18 @@ where
 {
 }
 
+//
+// impl Debug for ObjRef
+//
+impl<T> Debug for ObjRef<T>
+    where T: Obj + ?Sized + Send + Sync + 'static,
+{
+    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
+        read_obj!(let obj: &T = self);
+        Debug::fmt(obj, fmt)
+    }
+}
+
 //
 // impl Deref for ObjRef
 //
diff --git a/src/vm/error.rs b/src/vm/error.rs
index 4105658..e9580b3 100644
--- a/src/vm/error.rs
+++ b/src/vm/error.rs
@@ -1,9 +1,24 @@
+use crate::obj::prelude::*;
 use snafu::Snafu;
 
 #[derive(Debug, Snafu)]
 pub enum Error {
-    #[snafu(display("attempted to pop an empty stack"))]
-    EmptyStack,
+    #[snafu(display("missing attribute: {}", global_sym_lookup(*attr).unwrap()))]
+    MissingAttr {
+        attr: Sym,
+    },
+
+    #[snafu(display("{}", error))]
+    ValueError {
+        error: String,
+        value: ObjRef,
+    },
+
+    #[snafu(display("incorrect function arity; expected {} but got {} instead", expected, got))]
+    ArityError {
+        expected: usize,
+        got: usize,
+    },
 }
 
 pub type Result<T, E = Error> = std::result::Result<T, E>;
diff --git a/src/vm/mod.rs b/src/vm/mod.rs
index 21f96ec..1ad0c68 100644
--- a/src/vm/mod.rs
+++ b/src/vm/mod.rs
@@ -6,7 +6,7 @@ pub mod signal;
 
 use crate::{
     obj::{builtin::BUILTIN_OBJS, reserved::*, prelude::*},
-    vm::{consts::ConstPool, frame::*, inst::*, signal::*},
+    vm::{consts::ConstPool, error::*, frame::*, inst::*, signal::*},
 };
 
 #[derive(Debug)]
@@ -92,25 +92,26 @@ impl<'c> Vm<'c> {
     }
 
     /// Calls a function.
-    pub fn call(&mut self, fun: ObjRef, args: Vec<ObjRef>) -> ObjRef {
-        self.handle_signal(Signal::Call(fun, args));
-        self.resume_until_return();
-        self.pop().expect("return value")
+    pub fn call(&mut self, fun: ObjRef, args: Vec<ObjRef>) -> Result<ObjRef> {
+        self.handle_signal(Signal::Call(fun, args))?;
+        self.resume_until_return()?;
+        Ok(self.pop().expect("return value"))
     }
 
     /// Resumes execution of the current program.
-    pub fn resume(&mut self) {
+    pub fn resume(&mut self) -> Result<()> {
         while !self.frames().is_empty() {
-            self.resume_until_return();
+            self.resume_until_return()?;
         }
+        Ok(())
     }
 
     /// Resume execution until the currently executing function returns.
-    pub fn resume_until_return(&mut self) {
+    pub fn resume_until_return(&mut self) -> Result<()> {
         // resume execution until control is returned to the originally calling function
         let start_frames = self.frames().len();
         if start_frames == 0 {
-            return;
+            return Ok(());
         }
 
         while self.frames().len() >= start_frames {
@@ -119,14 +120,15 @@ impl<'c> Vm<'c> {
                 Frame::Native(fun) => {
                     let callee = fun.callee().clone();
                     let args = fun.args().clone();
-                    (*fun.fun_ptr())(callee, self, args)
+                    (*fun.fun_ptr())(callee, self, args)?
                 }
                 Frame::User(_) => {
                     self.resume_user_fun()
                 }
             };
-            self.handle_signal(signal);
+            self.handle_signal(signal)?;
         }
+        Ok(())
     }
 
     fn resume_user_fun(&mut self) -> Signal {
@@ -142,13 +144,13 @@ impl<'c> Vm<'c> {
     ///
     /// The signal may originate from the VM itself, or from an external location. Signals
     /// generally interrupt control flow of a program.
-    pub fn handle_signal(&mut self, signal: Signal) {
+    pub fn handle_signal(&mut self, signal: Signal) -> Result<()> {
         match signal {
             Signal::Call(callee, args) => {
                 read_obj!(let callee_obj = callee);
                 let frame = callee_obj.as_fun()
-                    .expect("callable function")
-                    .create_frame(callee.clone(), self, args);
+                    .ok_or_else(|| Error::ValueError { error: "cannot call this object".to_string(), value: callee.clone() })?
+                    .create_frame(callee.clone(), self, args)?;
                 // Jump to the first address of the new function call if it's a user function
                 if let Frame::User(_) = &frame {
                     self.set_pc(0);
@@ -171,6 +173,7 @@ impl<'c> Vm<'c> {
                 self.push(ret_val);
             }
         }
+        Ok(())
     }
 
     /// Gets the current instruction.