diff --git a/packages/language-core/lib/utils.ts b/packages/language-core/lib/utils.ts
index bfa4c839..b09150cf 100644
--- a/packages/language-core/lib/utils.ts
+++ b/packages/language-core/lib/utils.ts
@@ -6,7 +6,7 @@ export class FileMap<T> extends Map<string, T> {
 		super();
 	}
 
-	keys() {
+	keys(): IterableIterator<string> {
 		return this.originalFileNames.values();
 	}