diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index f6ca2f45d34..6ab0f5543ec 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2736,7 +2736,7 @@ namespace ts {
         getCompilerOptions(): CompilerOptions;
 
         getSourceFiles(): ReadonlyArray<SourceFile>;
-        getSourceFile(fileName: string): SourceFile;
+        getSourceFile(fileName: string): SourceFile | undefined;
         getResolvedTypeReferenceDirectives(): ReadonlyMap<ResolvedTypeReferenceDirective>;
     }