diff --git a/src/protocol/eval/executor.rs b/src/protocol/eval/executor.rs
index a3ff4fdb1d881b97dd0aebbfe8115871e5a5f51c..913be03fa88e408a2ac4bc3b71832f87bfbfaf8d 100644
--- a/src/protocol/eval/executor.rs
+++ b/src/protocol/eval/executor.rs
@@ -37,7 +37,7 @@ pub(crate) struct Frame {
 
 impl Frame {
     /// Creates a new execution frame. Does not modify the stack in any way.
-    pub fn new(heap: &Heap, definition_id: ProcedureDefinitionId, monomorph_type_id: TypeId) -> Self {
+    pub fn new(heap: &Heap, definition_id: ProcedureDefinitionId, monomorph_type_id: TypeId, monomorph_index: u32) -> Self {
         let definition = &heap[definition_id];
         let outer_scope_id = definition.scope;
         let first_statement_id = definition.body;
@@ -63,6 +63,7 @@ impl Frame {
         Frame{
             definition: definition_id,
             monomorph_type_id,
+            monomorph_index: monomorph_index as usize,
             position: first_statement_id.upcast(),
             expr_stack: VecDeque::with_capacity(128),
             expr_values: VecDeque::with_capacity(128),
@@ -222,14 +223,15 @@ pub struct Prompt {
 }
 
 impl Prompt {
-    pub fn new(_types: &TypeTable, heap: &Heap, def: ProcedureDefinitionId, type_id: TypeId, args: ValueGroup) -> Self {
+    pub fn new(types: &TypeTable, heap: &Heap, def: ProcedureDefinitionId, type_id: TypeId, args: ValueGroup) -> Self {
         let mut prompt = Self{
             frames: Vec::new(),
             store: Store::new(),
         };
 
         // Maybe do typechecking in the future?
-        let new_frame = Frame::new(heap, def, type_id);
+        let monomorph_index = types.get_monomorph(type_id).variant.as_procedure().monomorph_index;
+        let new_frame = Frame::new(heap, def, type_id, monomorph_index);
         let max_stack_size = new_frame.max_stack_size;
         prompt.frames.push(new_frame);
         args.into_store(&mut prompt.store);
@@ -767,10 +769,10 @@ impl Prompt {
 
                                     // Determine the monomorph index of the function we're calling
                                     let mono_data = &heap[cur_frame.definition].monomorphs[cur_frame.monomorph_index];
-                                    let type_id = mono_data.expr_info[expr.type_index as usize].variant.as_procedure().0;
+                                    let (type_id, monomorph_index) = mono_data.expr_info[expr.type_index as usize].variant.as_procedure();
 
                                     // Push the new frame and reserve its stack size
-                                    let new_frame = Frame::new(heap, expr.procedure, type_id);
+                                    let new_frame = Frame::new(heap, expr.procedure, type_id, monomorph_index);
                                     let new_stack_size = new_frame.max_stack_size;
                                     self.frames.push(new_frame);
                                     self.store.cur_stack_boundary = new_stack_boundary;
@@ -1029,7 +1031,7 @@ impl Prompt {
                 );
 
                 let mono_data = &heap[cur_frame.definition].monomorphs[cur_frame.monomorph_index];
-                let type_id = mono_data.expr_info[expr.type_index as usize].variant.as_procedure().0;
+                let type_id = mono_data.expr_info[call_expr.type_index as usize].variant.as_procedure().0;
 
                 // Note that due to expression value evaluation they exist in
                 // reverse order on the stack.