diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 834122b3584..d481e72b689 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -969,11 +969,11 @@ namespace ts {
             return emitResult;
         }
 
-        function getSourceFile(fileName: string): SourceFile {
+        function getSourceFile(fileName: string): SourceFile | undefined {
             return getSourceFileByPath(toPath(fileName, currentDirectory, getCanonicalFileName));
         }
 
-        function getSourceFileByPath(path: Path): SourceFile {
+        function getSourceFileByPath(path: Path): SourceFile | undefined {
             return filesByName.get(path);
         }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index ed1d5dcbe23..da438809d92 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2352,8 +2352,8 @@ namespace ts {
 
     export interface ScriptReferenceHost {
         getCompilerOptions(): CompilerOptions;
-        getSourceFile(fileName: string): SourceFile;
-        getSourceFileByPath(path: Path): SourceFile;
+        getSourceFile(fileName: string): SourceFile | undefined;
+        getSourceFileByPath(path: Path): SourceFile | undefined;
         getCurrentDirectory(): string;
     }