diff --git a/compiler/gen/src/llvm/refcounting.rs b/compiler/gen/src/llvm/refcounting.rs
index d3099dd145..e68f788040 100644
--- a/compiler/gen/src/llvm/refcounting.rs
+++ b/compiler/gen/src/llvm/refcounting.rs
@@ -546,20 +546,13 @@ fn modify_refcount_layout_help<'a, 'ctx, 'env>(
         }
         Closure(_, lambda_set, _) => {
             if lambda_set.contains_refcounted() {
-                let wrapper_struct = value.into_struct_value();
-
-                let field_ptr = env
-                    .builder
-                    .build_extract_value(wrapper_struct, 1, "modify_rc_closure_data")
-                    .unwrap();
-
                 modify_refcount_layout_help(
                     env,
                     parent,
                     layout_ids,
                     mode,
                     when_recursive,
-                    field_ptr,
+                    value,
                     &lambda_set.runtime_representation(),
                 )
             }
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index c81039d5d2..e686e83183 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -3950,9 +3950,6 @@ pub fn with_hole<'a>(
                             if let Layout::Closure(_, lambda_set, _) = full_layout {
                                 let closure_data_symbol = function_symbol;
 
-                                // layout of the closure record
-                                let closure_data_layout = lambda_set.runtime_representation();
-
                                 result = match_on_lambda_set(
                                     env,
                                     lambda_set,
@@ -3988,9 +3985,6 @@ pub fn with_hole<'a>(
                                 Layout::Closure(_, lambda_set, _) => {
                                     let closure_data_symbol = env.unique_symbol();
 
-                                    // layout of the closure record
-                                    let closure_data_layout = lambda_set.runtime_representation();
-
                                     result = match_on_lambda_set(
                                         env,
                                         lambda_set,
@@ -4110,7 +4104,28 @@ pub fn with_hole<'a>(
                     let list_symbol = arg_symbols[0];
                     let closure_data_symbol = arg_symbols[1];
 
-                    panic!()
+                    let closure_data_layout = return_on_layout_error!(
+                        env,
+                        layout_cache.from_var(env.arena, args[1].0, env.subs)
+                    );
+
+                    let arena = env.arena;
+
+                    match closure_data_layout {
+                        Layout::Closure(_, lambda_set, _) => lowlevel_match_on_lambda_set(
+                            env,
+                            lambda_set,
+                            closure_data_symbol,
+                            |top_level_function, closure_data| self::Call {
+                                call_type: CallType::LowLevel { op },
+                                arguments: arena.alloc([list_symbol, top_level_function]),
+                            },
+                            layout,
+                            assigned,
+                            hole,
+                        ),
+                        _ => unreachable!(),
+                    }
                 }
                 _ => {
                     let call = self::Call {
@@ -5923,7 +5938,6 @@ fn call_by_name_help<'a>(
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
-    dbg!(proc_name, &loc_args);
     let original_fn_var = fn_var;
     let arena = env.arena;
 
@@ -7378,18 +7392,78 @@ pub fn num_argument_to_int_or_float(
     }
 }
 
-fn lambda_set_lowlevel<'a, F>(
+/// Use the lambda set to figure out how to make a lowlevel call
+fn lowlevel_match_on_lambda_set<'a, F>(
     env: &mut Env<'a, '_>,
-    lambda_set: &'a [(TagName, &'a [Layout<'a>])],
-    to_lowlevel_call: F,
-    closure_tag_id_symbol: Symbol,
+    lambda_set: LambdaSet<'a>,
     closure_data_symbol: Symbol,
+    to_lowlevel_call: F,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    F: Fn(Symbol) -> Stmt<'a>,
+    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+{
+    match lambda_set.runtime_representation() {
+        Layout::Union(_) => {
+            let closure_tag_id_symbol = env.unique_symbol();
+
+            let result = lowlevel_union_lambda_set_to_switch(
+                env,
+                lambda_set.set,
+                closure_tag_id_symbol,
+                Layout::Builtin(crate::layout::TAG_SIZE),
+                closure_data_symbol,
+                to_lowlevel_call,
+                return_layout,
+                assigned,
+                hole,
+            );
+
+            // extract & assign the closure_tag_id_symbol
+            let expr = Expr::AccessAtIndex {
+                index: 0,
+                field_layouts: env.arena.alloc([Layout::Builtin(Builtin::Int64)]),
+                structure: closure_data_symbol,
+                wrapped: Wrapped::MultiTagUnion,
+            };
+
+            Stmt::Let(
+                closure_tag_id_symbol,
+                expr,
+                Layout::Builtin(Builtin::Int64),
+                env.arena.alloc(result),
+            )
+        }
+        Layout::Struct(_) => {
+            let function_symbol = lambda_set.set[0].0;
+
+            // build the call
+            Stmt::Let(
+                assigned,
+                Expr::Call(to_lowlevel_call(function_symbol, closure_data_symbol)),
+                return_layout,
+                env.arena.alloc(hole),
+            )
+        }
+        other => todo!("{:?}", other),
+    }
+}
+
+fn lowlevel_union_lambda_set_to_switch<'a, F>(
+    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,
+    return_layout: Layout<'a>,
+    assigned: Symbol,
+    hole: &'a Stmt<'a>,
+) -> Stmt<'a>
+where
+    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
 {
     debug_assert!(!lambda_set.is_empty());
 
@@ -7397,13 +7471,19 @@ where
 
     let mut branches = Vec::with_capacity_in(lambda_set.len(), env.arena);
 
-    for (i, (tag_name, _)) in lambda_set.iter().enumerate() {
-        if let TagName::Closure(function_symbol) = tag_name {
-            let stmt = to_lowlevel_call(*function_symbol);
-            branches.push((i as u64, BranchInfo::None, stmt));
-        } else {
-            unreachable!("non-closure tag in lambda set")
-        }
+    for (i, (function_symbol, _)) in lambda_set.iter().enumerate() {
+        let assigned = env.unique_symbol();
+
+        let hole = Stmt::Jump(join_point_id, env.arena.alloc([assigned]));
+
+        // build the call
+        let stmt = Stmt::Let(
+            assigned,
+            Expr::Call(to_lowlevel_call(*function_symbol, closure_data_symbol)),
+            return_layout,
+            env.arena.alloc(hole),
+        );
+        branches.push((i as u64, BranchInfo::None, stmt));
     }
 
     let default_branch = {
@@ -7414,7 +7494,7 @@ where
 
     let switch = Stmt::Switch {
         cond_symbol: closure_tag_id_symbol,
-        cond_layout: Layout::Builtin(Builtin::Int64),
+        cond_layout: closure_tag_id_layout,
         branches: branches.into_bump_slice(),
         default_branch,
         ret_layout: return_layout,
@@ -7434,6 +7514,31 @@ where
     }
 }
 
+fn lowlevel_union_lambda_set_branch<'a, F>(
+    env: &mut Env<'a, '_>,
+    join_point_id: JoinPointId,
+    function_symbol: Symbol,
+    closure_data_symbol: Symbol,
+    closure_data_layout: Layout<'a>,
+    to_lowlevel_call: F,
+    return_layout: Layout<'a>,
+) -> Stmt<'a>
+where
+    F: Fn(Symbol, Symbol) -> Call<'a> + Copy,
+{
+    let assigned = env.unique_symbol();
+
+    let hole = Stmt::Jump(join_point_id, env.arena.alloc([assigned]));
+
+    // build the call
+    Stmt::Let(
+        assigned,
+        Expr::Call(to_lowlevel_call(function_symbol, closure_data_symbol)),
+        return_layout,
+        env.arena.alloc(hole),
+    )
+}
+
 /// Use the lambda set to figure out how to make a call-by-name
 fn match_on_lambda_set<'a>(
     env: &mut Env<'a, '_>,
@@ -7452,6 +7557,7 @@ fn match_on_lambda_set<'a>(
             let result = union_lambda_set_to_switch(
                 env,
                 lambda_set.set,
+                lambda_set.runtime_representation(),
                 closure_tag_id_symbol,
                 Layout::Builtin(crate::layout::TAG_SIZE),
                 closure_data_symbol,
@@ -7480,7 +7586,7 @@ fn match_on_lambda_set<'a>(
         Layout::Struct(fields) => {
             let function_symbol = lambda_set.set[0].0;
 
-            lambda_set_to_switch_make_branch_help(
+            union_lambda_set_branch_help(
                 env,
                 function_symbol,
                 closure_data_symbol,
@@ -7531,6 +7637,7 @@ fn match_on_lambda_set<'a>(
 fn union_lambda_set_to_switch<'a>(
     env: &mut Env<'a, '_>,
     lambda_set: &'a [(Symbol, &'a [Layout<'a>])],
+    closure_layout: Layout<'a>,
     closure_tag_id_symbol: Symbol,
     closure_tag_id_layout: Layout<'a>,
     closure_data_symbol: Symbol,
@@ -7546,16 +7653,8 @@ fn union_lambda_set_to_switch<'a>(
 
     let mut branches = Vec::with_capacity_in(lambda_set.len(), env.arena);
 
-    let closure_layout = {
-        let mut temporary = Vec::with_capacity_in(lambda_set.len(), env.arena);
-
-        temporary.extend(lambda_set.iter().map(|x| x.1));
-
-        Layout::Union(UnionLayout::NonRecursive(temporary.into_bump_slice()))
-    };
-
     for (i, (function_symbol, _)) in lambda_set.iter().enumerate() {
-        let stmt = lambda_set_to_switch_make_branch(
+        let stmt = union_lambda_set_branch(
             env,
             join_point_id,
             *function_symbol,
@@ -7596,7 +7695,7 @@ fn union_lambda_set_to_switch<'a>(
     }
 }
 
-fn lambda_set_to_switch_make_branch<'a>(
+fn union_lambda_set_branch<'a>(
     env: &mut Env<'a, '_>,
     join_point_id: JoinPointId,
     function_symbol: Symbol,
@@ -7610,7 +7709,7 @@ fn lambda_set_to_switch_make_branch<'a>(
 
     let hole = Stmt::Jump(join_point_id, env.arena.alloc([result_symbol]));
 
-    lambda_set_to_switch_make_branch_help(
+    union_lambda_set_branch_help(
         env,
         function_symbol,
         closure_data_symbol,
@@ -7623,7 +7722,7 @@ fn lambda_set_to_switch_make_branch<'a>(
     )
 }
 
-fn lambda_set_to_switch_make_branch_help<'a>(
+fn union_lambda_set_branch_help<'a>(
     env: &mut Env<'a, '_>,
     function_symbol: Symbol,
     closure_data_symbol: Symbol,
diff --git a/compiler/test_gen/src/gen_primitives.rs b/compiler/test_gen/src/gen_primitives.rs
index cae3c5c862..aff0853438 100644
--- a/compiler/test_gen/src/gen_primitives.rs
+++ b/compiler/test_gen/src/gen_primitives.rs
@@ -992,18 +992,19 @@ fn specialize_closure() {
 
             foo = \{} ->
                 x = 41
-                y = 1
+                y = [1]
 
                 f = \{} -> x
-                g = \{} -> x + y
+                g = \{} -> x + List.len y
 
                 [ f, g ]
 
+            apply = \f -> f {}
+
             main =
                 items = foo {}
 
-                # List.len items
-                List.map items (\f -> f {})
+                List.map items apply 
             "#
         ),
         RocList::from_slice(&[41, 42]),