diff --git a/src/compiler/scanner.ts b/src/compiler/scanner.ts
index a03bf733f75..da7308ecb89 100644
--- a/src/compiler/scanner.ts
+++ b/src/compiler/scanner.ts
@@ -734,11 +734,11 @@ namespace ts {
         return comments;
     }
 
-    export function getLeadingCommentRanges(text: string, pos: number): CommentRange[] {
+    export function getLeadingCommentRanges(text: string, pos: number): CommentRange[] | undefined {
         return reduceEachLeadingCommentRange(text, pos, appendCommentRange, undefined, undefined);
     }
 
-    export function getTrailingCommentRanges(text: string, pos: number): CommentRange[] {
+    export function getTrailingCommentRanges(text: string, pos: number): CommentRange[] | undefined {
         return reduceEachTrailingCommentRange(text, pos, appendCommentRange, undefined, undefined);
     }