diff --git a/src/layout/store.zig b/src/layout/store.zig
index 7f6b6a6ea6..b82e34aae9 100644
--- a/src/layout/store.zig
+++ b/src/layout/store.zig
@@ -61,6 +61,9 @@ pub const Store = struct {
     // Reusable work stack for addTypeVar (so it can be stack-safe instead of recursing)
     work: work.Work,
 
+    // Cached List ident to avoid repeated string lookups (null if List doesn't exist in this env)
+    list_ident: ?Ident.Idx,
+
     // Number of primitive types that are pre-populated in the layout store
     // Must be kept in sync with the sentinel values in layout.zig Idx enum
     const num_scalars = 16;
@@ -144,6 +147,7 @@ pub const Store = struct {
             .tuple_data = try collections.SafeList(TupleData).initCapacity(env.gpa, 256),
             .layouts_by_var = layouts_by_var,
             .work = try Work.initCapacity(env.gpa, 32),
+            .list_ident = null, // Lazily initialized on first use
         };
     }
 
@@ -882,13 +886,19 @@ pub const Store = struct {
                     },
                     .nominal_type => |nominal_type| {
                         // Special handling for Builtin.List
-                        const list_ident = self.env.common.findIdent("List");
-                        if (list_ident != null and nominal_type.ident.ident_idx == list_ident.?) {
+                        // Lazily initialize list_ident if not yet cached
+                        if (self.list_ident == null) {
+                            self.list_ident = self.env.common.findIdent("List");
+                        }
+                        const is_builtin_list = if (self.list_ident) |list_ident|
+                            nominal_type.origin_module == self.env.builtin_module_ident and
+                                nominal_type.ident.ident_idx == list_ident
+                        else
+                            false;
+                        if (is_builtin_list) {
                             // Extract the element type from the type arguments
                             const type_args = self.types_store.sliceNominalArgs(nominal_type);
-                            if (type_args.len != 1) {
-                                @panic("List nominal type must have exactly 1 type parameter");
-                            }
+                            std.debug.assert(type_args.len == 1); // List must have exactly 1 type parameter
                             const elem_var = type_args[0];
 
                             // Check if the element type is unbound (flex or rigid)
diff --git a/src/layout/store_test.zig b/src/layout/store_test.zig
index 71fc5ed24e..4ac09278ae 100644
--- a/src/layout/store_test.zig
+++ b/src/layout/store_test.zig
@@ -465,8 +465,8 @@ test "nested ZST detection - List of record with ZST field" {
     // List of this record should be list_of_zst since the record only has ZST fields
     const list_ident_idx = try lt.module_env.getIdentStore().insert(lt.module_env.gpa, Ident.for_text("List"));
     const list_type_ident = types.TypeIdent{ .ident_idx = list_ident_idx };
-    const origin_module = Ident.Idx{ .attributes = .{ .effectful = false, .ignored = false, .reassignable = false }, .idx = 0 };
-    const list_content = try lt.type_store.mkNominal(list_type_ident, record_var, &[_]types.Var{record_var}, origin_module);
+    const builtin_module_idx = try lt.module_env.insertIdent(Ident.for_text("Builtin"));
+    const list_content = try lt.type_store.mkNominal(list_type_ident, record_var, &[_]types.Var{record_var}, builtin_module_idx);
     const list_var = try lt.type_store.freshFromContent(list_content);
     const list_idx = try lt.layout_store.addTypeVar(list_var, &lt.type_scope);
     try testing.expect(lt.layout_store.getLayout(list_idx).tag == .list_of_zst);
@@ -530,8 +530,8 @@ test "nested ZST detection - deeply nested" {
     // List({ field: ({ field2: {} }, ()) })
     const list_ident_idx = try lt.module_env.getIdentStore().insert(lt.module_env.gpa, Ident.for_text("List"));
     const list_type_ident = types.TypeIdent{ .ident_idx = list_ident_idx };
-    const origin_module = Ident.Idx{ .attributes = .{ .effectful = false, .ignored = false, .reassignable = false }, .idx = 0 };
-    const list_content = try lt.type_store.mkNominal(list_type_ident, outer_record_var, &[_]types.Var{outer_record_var}, origin_module);
+    const builtin_module_idx = try lt.module_env.insertIdent(Ident.for_text("Builtin"));
+    const list_content = try lt.type_store.mkNominal(list_type_ident, outer_record_var, &[_]types.Var{outer_record_var}, builtin_module_idx);
     const list_var = try lt.type_store.freshFromContent(list_content);
     const list_idx = try lt.layout_store.addTypeVar(list_var, &lt.type_scope);