diff --git a/dust-lang/benches/addictive_addition.rs b/dust-lang/benches/addictive_addition.rs
index ffb5264..f6b121e 100644
--- a/dust-lang/benches/addictive_addition.rs
+++ b/dust-lang/benches/addictive_addition.rs
@@ -1,7 +1,7 @@
 use std::time::Duration;
 
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use dust_lang::{run, DustString};
+use dust_lang::run;
 
 const SOURCE: &str = r"
     let mut i = 0
@@ -12,7 +12,7 @@ const SOURCE: &str = r"
 ";
 
 fn addictive_addition(source: &str) {
-    run(Some(DustString::from("addictive_addition")), source).unwrap();
+    run(source).unwrap();
 }
 
 fn criterion_benchmark(c: &mut Criterion) {
diff --git a/dust-lang/benches/fibonacci.rs b/dust-lang/benches/fibonacci.rs
index da88346..cd83319 100644
--- a/dust-lang/benches/fibonacci.rs
+++ b/dust-lang/benches/fibonacci.rs
@@ -1,7 +1,7 @@
 use std::time::Duration;
 
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use dust_lang::{run, DustString};
+use dust_lang::run;
 
 const SOURCE: &str = r"
     fn fib (n: int) -> int {
@@ -15,7 +15,7 @@ const SOURCE: &str = r"
 ";
 
 fn addictive_addition(source: &str) {
-    run(Some(DustString::from("fibonacci")), source).unwrap();
+    run(source).unwrap();
 }
 
 fn criterion_benchmark(c: &mut Criterion) {
diff --git a/dust-lang/src/compiler/mod.rs b/dust-lang/src/compiler/mod.rs
index d9f88f1..4358839 100644
--- a/dust-lang/src/compiler/mod.rs
+++ b/dust-lang/src/compiler/mod.rs
@@ -49,10 +49,10 @@ use crate::{
 ///
 /// assert_eq!(chunk.instructions().len(), 3);
 /// ```
-pub fn compile(program_name: Option<DustString>, source: &str) -> Result<Chunk, DustError> {
+pub fn compile(source: &str) -> Result<Chunk, DustError> {
     let lexer = Lexer::new(source);
-    let mut compiler = Compiler::new(lexer, program_name, true)
-        .map_err(|error| DustError::compile(error, source))?;
+    let mut compiler =
+        Compiler::new(lexer, None, true).map_err(|error| DustError::compile(error, source))?;
 
     compiler
         .compile()
diff --git a/dust-lang/src/instruction/mod.rs b/dust-lang/src/instruction/mod.rs
index e0a32e7..8cb0234 100644
--- a/dust-lang/src/instruction/mod.rs
+++ b/dust-lang/src/instruction/mod.rs
@@ -768,7 +768,7 @@ impl Debug for Instruction {
 
 impl Display for Instruction {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "{} {}", self.operation(), self.disassembly_info())
+        write!(f, "{} | {}", self.operation(), self.disassembly_info())
     }
 }
 
diff --git a/dust-lang/src/vm/mod.rs b/dust-lang/src/vm/mod.rs
index cb5336f..fec393c 100644
--- a/dust-lang/src/vm/mod.rs
+++ b/dust-lang/src/vm/mod.rs
@@ -18,10 +18,10 @@ pub use thread::{Thread, ThreadData};
 
 use tracing::{span, Level};
 
-use crate::{compile, Chunk, DustError, DustString, Value};
+use crate::{compile, Chunk, DustError, Value};
 
-pub fn run(program_name: Option<DustString>, source: &str) -> Result<Option<Value>, DustError> {
-    let chunk = compile(program_name, source)?;
+pub fn run(source: &str) -> Result<Option<Value>, DustError> {
+    let chunk = compile(source)?;
     let vm = Vm::new(chunk);
 
     Ok(vm.run())
@@ -92,8 +92,8 @@ pub enum Pointer {
 impl Display for Pointer {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         match self {
-            Self::Stack(index) => write!(f, "R{}", index),
-            Self::Constant(index) => write!(f, "C{}", index),
+            Self::Stack(index) => write!(f, "PR{}", index),
+            Self::Constant(index) => write!(f, "PC{}", index),
         }
     }
 }
diff --git a/dust-lang/src/vm/run_action.rs b/dust-lang/src/vm/run_action.rs
index 6cf5234..be1ae38 100644
--- a/dust-lang/src/vm/run_action.rs
+++ b/dust-lang/src/vm/run_action.rs
@@ -544,11 +544,17 @@ pub fn call(instruction: Instruction, data: &mut ThreadData) -> bool {
         ip: current_call.ip,
         record: Record::new(prototype),
     };
+    let mut argument_index = 0;
 
-    for (argument_index, register_index) in (first_argument_register..return_register).enumerate() {
-        let argument = current_call
+    for register_index in first_argument_register..return_register {
+        let value_option = current_call
             .record
-            .clone_register_value_or_constant_unchecked(register_index);
+            .open_register_allow_empty_unchecked(register_index);
+        let argument = if let Some(value) = value_option {
+            value.clone()
+        } else {
+            continue;
+        };
 
         trace!(
             "Passing argument \"{argument}\" to {}",
@@ -560,7 +566,9 @@ pub fn call(instruction: Instruction, data: &mut ThreadData) -> bool {
 
         next_call
             .record
-            .set_register(argument_index as u8, Register::Value(argument));
+            .set_register(argument_index, Register::Value(argument));
+
+        argument_index += 1;
     }
 
     data.next_action = get_next_action(&mut next_call.record);
@@ -601,12 +609,12 @@ pub fn r#return(instruction: Instruction, data: &mut ThreadData) -> bool {
     };
 
     let outer_call = data.call_stack.last_mut_unchecked();
-    let destination = current_call.return_register;
 
     if should_return_value {
         let return_value = current_call
             .record
             .empty_register_or_clone_constant_unchecked(return_register);
+        let destination = current_call.return_register;
 
         outer_call
             .record