diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 5347d7caaf..4e5ca9f07e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4095,8 +4095,8 @@ namespace ts {
     }
 
     export interface CompilerHost extends ModuleResolutionHost {
-        getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile;
-        getSourceFileByPath?(fileName: string, path: Path, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile;
+        getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile | undefined;
+        getSourceFileByPath?(fileName: string, path: Path, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile | undefined;
         getCancellationToken?(): CancellationToken;
         getDefaultLibFileName(options: CompilerOptions): string;
         getDefaultLibLocation?(): string;