diff --git a/packages/core/src/material-library.test.ts b/packages/core/src/material-library.test.ts
new file mode 100644
index 00000000..344cbbb1
--- /dev/null
+++ b/packages/core/src/material-library.test.ts
@@ -0,0 +1,13 @@
+import { describe, expect, test } from 'bun:test'
+import { getLibraryMaterialIdFromRef, getSceneMaterialIdFromRef } from './material-library'
+
+describe('material references', () => {
+  test('rejects malformed runtime values instead of calling string methods', () => {
+    const malformedRefs: unknown[] = [42, true, {}, []]
+
+    for (const ref of malformedRefs) {
+      expect(getLibraryMaterialIdFromRef(ref as string)).toBeNull()
+      expect(getSceneMaterialIdFromRef(ref as string)).toBeNull()
+    }
+  })
+})
diff --git a/packages/core/src/material-library.ts b/packages/core/src/material-library.ts
index f887d36b..e45a4887 100644
--- a/packages/core/src/material-library.ts
+++ b/packages/core/src/material-library.ts
@@ -4170,13 +4170,14 @@ export function toSceneMaterialRef(id: string) {
 }
 
 export function getLibraryMaterialIdFromRef(materialRef?: string | null) {
-  if (!materialRef) return null
+  if (typeof materialRef !== 'string') return null
   if (!materialRef.startsWith(LIBRARY_MATERIAL_REF_PREFIX)) return null
   return materialRef.slice(LIBRARY_MATERIAL_REF_PREFIX.length)
 }
 
 export function getSceneMaterialIdFromRef(materialRef?: string | null): string | null {
-  if (!materialRef?.startsWith(SCENE_MATERIAL_REF_PREFIX)) return null
+  if (typeof materialRef !== 'string') return null
+  if (!materialRef.startsWith(SCENE_MATERIAL_REF_PREFIX)) return null
   return materialRef.slice(SCENE_MATERIAL_REF_PREFIX.length)
 }