diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index ada8a0510a..cf7c03556f 100755
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -1179,11 +1179,11 @@ namespace ts {
             return emitResult;
         }
 
-        function getSourceFile(fileName: string): SourceFile {
+        function getSourceFile(fileName: string): SourceFile | undefined {
             return getSourceFileByPath(toPath(fileName));
         }
 
-        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 e87069d24e..1874821727 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2466,8 +2466,8 @@ 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;
     }
 
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 9b2270d29e..a3499de08a 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -1609,8 +1609,8 @@ declare namespace ts {
     }
     interface ScriptReferenceHost {
         getCompilerOptions(): CompilerOptions;
-        getSourceFile(fileName: string): SourceFile;
-        getSourceFileByPath(path: Path): SourceFile;
+        getSourceFile(fileName: string): SourceFile | undefined;
+        getSourceFileByPath(path: Path): SourceFile | undefined;
         getCurrentDirectory(): string;
     }
     interface ParseConfigHost {
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 6ef259ddc8..98d7137dde 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -1609,8 +1609,8 @@ declare namespace ts {
     }
     interface ScriptReferenceHost {
         getCompilerOptions(): CompilerOptions;
-        getSourceFile(fileName: string): SourceFile;
-        getSourceFileByPath(path: Path): SourceFile;
+        getSourceFile(fileName: string): SourceFile | undefined;
+        getSourceFileByPath(path: Path): SourceFile | undefined;
         getCurrentDirectory(): string;
     }
     interface ParseConfigHost {