diff --git a/src/services/types.ts b/src/services/types.ts
index 80dac58095c..72ded445dba 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -212,9 +212,9 @@ namespace ts {
          *
          * If this is implemented, `getResolvedModuleWithFailedLookupLocationsFromCache` should be too.
          */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): ResolvedModule[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
         getResolvedModuleWithFailedLookupLocationsFromCache?(modulename: string, containingFile: string): ResolvedModuleWithFailedLookupLocations | undefined;
-        resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): ResolvedTypeReferenceDirective[];
+        resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
         /* @internal */ hasInvalidatedResolution?: HasInvalidatedResolution;
         /* @internal */ hasChangedAutomaticTypeDirectiveNames?: boolean;