diff --git a/src/ast/compiler/optimizer/engine.rs b/src/ast/compiler/optimizer/engine.rs
index 5ced0a1..6d6c95c 100644
--- a/src/ast/compiler/optimizer/engine.rs
+++ b/src/ast/compiler/optimizer/engine.rs
@@ -249,7 +249,7 @@ impl Optimizer {
                         && path.enter_lambda(&callee_opt.identity)
                     {
                         path.inlining_depth += 1;
-                        let mut inner_sub = sub.new_inner();
+                        let mut inner_sub = sub.new_for_inlining();
                         let collapsed = if inliner
                             .prepare_beta_reduction(params, &arg_nodes, body, &mut inner_sub)
                             .is_some()
@@ -286,7 +286,7 @@ impl Optimizer {
                             && positional_count.is_some()
                             && !lambda_node.ty.is_recursive
                         {
-                            let mut inner_sub = sub.new_inner();
+                            let mut inner_sub = sub.new_for_inlining();
                             path.inlining_stack.insert(*idx);
                             path.inlining_depth += 1;
                             let collapsed = if inliner
@@ -314,7 +314,7 @@ impl Optimizer {
                         && !closure.function_node.ty.is_recursive
                         && path.enter_lambda(&closure.function_node.identity)
                     {
-                        let mut closure_sub = SubstitutionMap::new();
+                        let mut closure_sub = sub.new_for_inlining();
                         for (i, cell) in closure.upvalues.iter().enumerate() {
                             closure_sub.add_value(
                                 Address::Upvalue(UpvalueIdx(i as u32)),
@@ -323,22 +323,16 @@ impl Optimizer {
                         }
 
                         path.inlining_depth += 1;
-                        let inlined_body = self.visit_node(
-                            closure.function_node.clone(),
-                            &mut closure_sub,
-                            path,
-                        );
-
                         let collapsed = if inliner
                             .prepare_beta_reduction(
                                 &closure.parameter_node,
                                 &arg_nodes,
-                                &inlined_body,
+                                &closure.function_node,
                                 &mut closure_sub,
                             )
                             .is_some()
                         {
-                            Some(self.visit_node(inlined_body, &mut closure_sub, path))
+                            Some(self.visit_node(closure.function_node.clone(), &mut closure_sub, path))
                         } else {
                             None
                         };
diff --git a/src/ast/compiler/optimizer/inliner.rs b/src/ast/compiler/optimizer/inliner.rs
index 8ba1ccd..90a5ad4 100644
--- a/src/ast/compiler/optimizer/inliner.rs
+++ b/src/ast/compiler/optimizer/inliner.rs
@@ -128,8 +128,7 @@ impl<'a> Inliner<'a> {
                 }
 
                 if let Address::Local(slot) = addr {
-                    sub.slot_mapping.insert(*slot, *slot);
-                    sub.next_slot = sub.next_slot.max(slot.0 + 1);
+                    sub.map_slot(*slot);
                 }
                 *offset += 1;
             }
diff --git a/src/ast/compiler/optimizer/substitution_map.rs b/src/ast/compiler/optimizer/substitution_map.rs
index 94315ce..0a5b988 100644
--- a/src/ast/compiler/optimizer/substitution_map.rs
+++ b/src/ast/compiler/optimizer/substitution_map.rs
@@ -38,6 +38,12 @@ impl SubstitutionMap {
         inner
     }
 
+    pub fn new_for_inlining(&self) -> Self {
+        let mut inner = self.new_inner();
+        inner.next_slot = self.next_slot;
+        inner
+    }
+
     pub fn add_ast_substitution(&mut self, addr: Address, node: AnalyzedNode) {
         self.ast_substitutions.insert(addr, Rc::new(node));
     }