diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index b08d897b35c23..017d3545206bc 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -2909,7 +2909,7 @@ namespace ts {
             return value;
         }
 
-        export function assertEachDefined<T, A extends ReadonlyArray<T>>(value: A, message: string): A {
+        export function assertEachDefined<T, A extends ReadonlyArray<T>>(value: A, message?: string): A {
             for (const v of value) {
                 assertDefined(v, message);
             }
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index b76f8d8b154b7..3de4daa0f9b6b 100755
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -538,7 +538,7 @@ namespace ts {
         let resolveModuleNamesWorker: (moduleNames: string[], containingFile: string, reusedNames?: string[]) => ResolvedModuleFull[];
         const hasInvalidatedResolution = host.hasInvalidatedResolution || returnFalse;
         if (host.resolveModuleNames) {
-            resolveModuleNamesWorker = (moduleNames, containingFile, reusedNames) => host.resolveModuleNames(checkAllDefined(moduleNames), containingFile, reusedNames).map(resolved => {
+            resolveModuleNamesWorker = (moduleNames, containingFile, reusedNames) => host.resolveModuleNames(Debug.assertEachDefined(moduleNames), containingFile, reusedNames).map(resolved => {
                 // An older host may have omitted extension, in which case we should infer it from the file extension of resolvedFileName.
                 if (!resolved || (resolved as ResolvedModuleFull).extension !== undefined) {
                     return resolved as ResolvedModuleFull;
@@ -551,16 +551,16 @@ namespace ts {
         else {
             moduleResolutionCache = createModuleResolutionCache(currentDirectory, x => host.getCanonicalFileName(x));
             const loader = (moduleName: string, containingFile: string) => resolveModuleName(moduleName, containingFile, options, host, moduleResolutionCache).resolvedModule;
-            resolveModuleNamesWorker = (moduleNames, containingFile) => loadWithLocalCache(checkAllDefined(moduleNames), containingFile, loader);
+            resolveModuleNamesWorker = (moduleNames, containingFile) => loadWithLocalCache(Debug.assertEachDefined(moduleNames), containingFile, loader);
         }
 
         let resolveTypeReferenceDirectiveNamesWorker: (typeDirectiveNames: string[], containingFile: string) => ResolvedTypeReferenceDirective[];
         if (host.resolveTypeReferenceDirectives) {
-            resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile) => host.resolveTypeReferenceDirectives(checkAllDefined(typeDirectiveNames), containingFile);
+            resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile) => host.resolveTypeReferenceDirectives(Debug.assertEachDefined(typeDirectiveNames), containingFile);
         }
         else {
             const loader = (typesRef: string, containingFile: string) => resolveTypeReferenceDirective(typesRef, containingFile, options, host).resolvedTypeReferenceDirective;
-            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(checkAllDefined(typeReferenceDirectiveNames), containingFile, loader);
+            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(Debug.assertEachDefined(typeReferenceDirectiveNames), containingFile, loader);
         }
 
         // Map from a stringified PackageId to the source file with that id.
@@ -2415,11 +2415,6 @@ namespace ts {
         }
     }
 
-    function checkAllDefined(names: string[]): string[] {
-        Debug.assert(names.every(name => name !== undefined), "A name is undefined.", () => JSON.stringify(names));
-        return names;
-    }
-
     function getModuleNames({ imports, moduleAugmentations }: SourceFile): string[] {
         const res = imports.map(i => i.text);
         for (const aug of moduleAugmentations) {