diff --git a/compiler/gen/src/llvm/build.rs b/compiler/gen/src/llvm/build.rs
index c68ab4a66c..7fc8e29bcf 100644
--- a/compiler/gen/src/llvm/build.rs
+++ b/compiler/gen/src/llvm/build.rs
@@ -882,9 +882,10 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
                 .unwrap_or_else(|| panic!("LLVM error: Invalid call by pointer."))
         }
 
-        CallType::LowLevel { op } => {
-            run_low_level(env, layout_ids, scope, parent, layout, *op, arguments)
-        }
+        CallType::LowLevel {
+            op,
+            opt_closure_layout,
+        } => run_low_level(env, layout_ids, scope, parent, layout, *op, arguments),
 
         CallType::Foreign {
             foreign_symbol,
diff --git a/compiler/gen_dev/src/lib.rs b/compiler/gen_dev/src/lib.rs
index f7861bdc67..e86f50fe0d 100644
--- a/compiler/gen_dev/src/lib.rs
+++ b/compiler/gen_dev/src/lib.rs
@@ -211,9 +211,10 @@ where
                         }
                     }
 
-                    CallType::LowLevel { op: lowlevel } => {
-                        self.build_run_low_level(sym, lowlevel, arguments, layout)
-                    }
+                    CallType::LowLevel {
+                        op: lowlevel,
+                        opt_closure_layout: _,
+                    } => self.build_run_low_level(sym, lowlevel, arguments, layout),
                     x => Err(format!("the call type, {:?}, is not yet implemented", x)),
                 }
             }
diff --git a/compiler/mono/src/borrow.rs b/compiler/mono/src/borrow.rs
index d018d24556..6e5539cdb9 100644
--- a/compiler/mono/src/borrow.rs
+++ b/compiler/mono/src/borrow.rs
@@ -407,7 +407,10 @@ impl<'a> BorrowInfState<'a> {
                 self.own_args(arguments);
             }
 
-            LowLevel { op } => {
+            LowLevel {
+                op,
+                opt_closure_layout: _,
+            } => {
                 // very unsure what demand RunLowLevel should place upon its arguments
                 self.own_var(z);
 
diff --git a/compiler/mono/src/decision_tree.rs b/compiler/mono/src/decision_tree.rs
index d3bc2753d8..baa0ed17f5 100644
--- a/compiler/mono/src/decision_tree.rs
+++ b/compiler/mono/src/decision_tree.rs
@@ -1405,7 +1405,10 @@ fn compile_test_help<'a>(
     };
 
     let test = Expr::Call(crate::ir::Call {
-        call_type: crate::ir::CallType::LowLevel { op: LowLevel::Eq },
+        call_type: crate::ir::CallType::LowLevel {
+            op: LowLevel::Eq,
+            opt_closure_layout: None,
+        },
         arguments: arena.alloc([lhs, rhs]),
     });
 
diff --git a/compiler/mono/src/inc_dec.rs b/compiler/mono/src/inc_dec.rs
index 6cd24c7af6..47b8774259 100644
--- a/compiler/mono/src/inc_dec.rs
+++ b/compiler/mono/src/inc_dec.rs
@@ -442,7 +442,10 @@ impl<'a> Context<'a> {
         use crate::ir::CallType::*;
 
         match &call_type {
-            LowLevel { op } => {
+            LowLevel {
+                op,
+                opt_closure_layout: _,
+            } => {
                 let ps = crate::borrow::lowlevel_borrow_signature(self.arena, *op);
                 let b = self.add_dec_after_lowlevel(arguments, ps, b, b_live_vars);
 
@@ -762,7 +765,10 @@ impl<'a> Context<'a> {
 
                 use crate::ir::CallType;
                 let stmt = match &call.call_type {
-                    CallType::LowLevel { op } => {
+                    CallType::LowLevel {
+                        op,
+                        opt_closure_layout: _,
+                    } => {
                         let ps = crate::borrow::lowlevel_borrow_signature(self.arena, *op);
                         self.add_dec_after_lowlevel(call.arguments, ps, cont, &invoke_live_vars)
                     }
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index c1b414a2e0..cc1ce1e35e 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -1040,7 +1040,10 @@ impl<'a> Call<'a> {
                     .text("CallByPointer ")
                     .append(alloc.intersperse(it, " "))
             }
-            LowLevel { op: lowlevel } => {
+            LowLevel {
+                op: lowlevel,
+                opt_closure_layout: _,
+            } => {
                 let it = arguments.iter().map(|s| symbol_to_doc(alloc, *s));
 
                 alloc
@@ -1082,6 +1085,8 @@ pub enum CallType<'a> {
     },
     LowLevel {
         op: LowLevel,
+        /// the layout of the closure argument, if any
+        opt_closure_layout: Option<Layout<'a>>,
     },
 }
 
@@ -2671,8 +2676,8 @@ macro_rules! match_on_closure_argument {
                     $env,
                     lambda_set,
                     $closure_data_symbol,
-                    |top_level_function, closure_data| self::Call {
-                        call_type: CallType::LowLevel { op: $op },
+                    |top_level_function, closure_data, function_layout| self::Call {
+                        call_type: CallType::LowLevel { op: $op, opt_closure_layout: Some(function_layout) },
                         arguments: arena.alloc([$($x,)* top_level_function, closure_data]),
                     },
                     arena.alloc(top_level).full(),
@@ -4357,7 +4362,10 @@ pub fn with_hole<'a>(
                 }
                 _ => {
                     let call = self::Call {
-                        call_type: CallType::LowLevel { op },
+                        call_type: CallType::LowLevel {
+                            op,
+                            opt_closure_layout: None,
+                        },
                         arguments: arg_symbols,
                     };
 
@@ -4577,6 +4585,7 @@ pub fn from_can<'a>(
 
             let call_type = CallType::LowLevel {
                 op: LowLevel::ExpectTrue,
+                opt_closure_layout: None,
             };
             let arguments = env.arena.alloc([cond_symbol]);
             let call = self::Call {
@@ -7634,18 +7643,18 @@ pub fn num_argument_to_int_or_float(
 }
 
 /// Use the lambda set to figure out how to make a lowlevel call
-fn lowlevel_match_on_lambda_set<'a, F>(
+fn lowlevel_match_on_lambda_set<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     lambda_set: LambdaSet<'a>,
     closure_data_symbol: Symbol,
-    to_lowlevel_call: F,
+    to_lowlevel_call: ToLowLevelCall,
     function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
 {
     match lambda_set.runtime_representation() {
         Layout::Union(_) => {
@@ -7687,7 +7696,11 @@ where
             // build the call
             let stmt = Stmt::Let(
                 assigned,
-                Expr::Call(to_lowlevel_call(bound, closure_data_symbol)),
+                Expr::Call(to_lowlevel_call(
+                    bound,
+                    closure_data_symbol,
+                    function_layout,
+                )),
                 return_layout,
                 env.arena.alloc(hole),
             );
@@ -7736,20 +7749,20 @@ where
     }
 }
 
-fn lowlevel_union_lambda_set_to_switch<'a, F>(
+fn lowlevel_union_lambda_set_to_switch<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     lambda_set: &'a [(Symbol, &'a [Layout<'a>])],
     closure_tag_id_symbol: Symbol,
     closure_tag_id_layout: Layout<'a>,
     closure_data_symbol: Symbol,
-    to_lowlevel_call: F,
+    to_lowlevel_call: ToLowLevelCall,
     function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
 {
     debug_assert!(!lambda_set.is_empty());
 
@@ -7767,7 +7780,11 @@ where
         // build the call
         let stmt = Stmt::Let(
             assigned,
-            Expr::Call(to_lowlevel_call(bound, closure_data_symbol)),
+            Expr::Call(to_lowlevel_call(
+                bound,
+                closure_data_symbol,
+                function_layout,
+            )),
             return_layout,
             env.arena.alloc(hole),
         );
@@ -7810,18 +7827,18 @@ where
     }
 }
 
-fn lowlevel_union_lambda_set_branch<'a, F>(
+fn lowlevel_union_lambda_set_branch<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     join_point_id: JoinPointId,
     function_symbol: Symbol,
     closure_data_symbol: Symbol,
     closure_data_layout: Layout<'a>,
     function_layout: Layout<'a>,
-    to_lowlevel_call: F,
+    to_lowlevel_call: ToLowLevelCall,
     return_layout: Layout<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
 {
     let assigned = env.unique_symbol();
 
@@ -7832,7 +7849,11 @@ where
     // build the call
     let stmt = Stmt::Let(
         assigned,
-        Expr::Call(to_lowlevel_call(bound, closure_data_symbol)),
+        Expr::Call(to_lowlevel_call(
+            bound,
+            closure_data_symbol,
+            function_layout,
+        )),
         return_layout,
         env.arena.alloc(hole),
     );
@@ -8225,20 +8246,20 @@ fn enum_lambda_set_branch_help<'a>(
     )
 }
 
-fn lowlevel_enum_lambda_set_to_switch<'a, F>(
+fn lowlevel_enum_lambda_set_to_switch<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     lambda_set: &'a [(Symbol, &'a [Layout<'a>])],
     closure_tag_id_symbol: Symbol,
     closure_tag_id_layout: Layout<'a>,
     closure_data_symbol: Symbol,
-    to_lowlevel_call: F,
+    to_lowlevel_call: ToLowLevelCall,
     function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
 {
     debug_assert!(!lambda_set.is_empty());
 
@@ -8290,18 +8311,18 @@ where
     }
 }
 
-fn lowlevel_enum_lambda_set_branch<'a, F>(
+fn lowlevel_enum_lambda_set_branch<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     join_point_id: JoinPointId,
     function_symbol: Symbol,
     closure_data_symbol: Symbol,
     closure_data_layout: Layout<'a>,
-    to_lowlevel_call: F,
+    to_lowlevel_call: ToLowLevelCall,
     function_layout: Layout<'a>,
     return_layout: Layout<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
 {
     let result_symbol = env.unique_symbol();
 
@@ -8312,7 +8333,11 @@ where
     // build the call
     let stmt = Stmt::Let(
         result_symbol,
-        Expr::Call(to_lowlevel_call(bound, closure_data_symbol)),
+        Expr::Call(to_lowlevel_call(
+            bound,
+            closure_data_symbol,
+            function_layout,
+        )),
         return_layout,
         env.arena.alloc(hole),
     );