diff --git a/source-map.d.ts b/source-map.d.ts
index e4da538d..dd830fde 100644
--- a/source-map.d.ts
+++ b/source-map.d.ts
@@ -58,7 +58,7 @@ export interface NullableMappedPosition {
 }
 
 export interface MappingItem {
-  source: string;
+  source: string | null;
   generatedLine: number;
   generatedColumn: number;
   lastGeneratedColumn: number | null;