diff --git a/src/vs/editor/common/services/languagesRegistry.ts b/src/vs/editor/common/services/languagesRegistry.ts
index 38d14103a29..c734735a733 100644
--- a/src/vs/editor/common/services/languagesRegistry.ts
+++ b/src/vs/editor/common/services/languagesRegistry.ts
@@ -155,9 +155,9 @@ export class LanguagesRegistry extends Disposable {
 		if (Array.isArray(lang.extensions)) {
 			if (lang.configuration) {
 				// insert first as this appears to be the 'primary' language definition
-				resolvedLanguage.extensions.splice(0, 0, ...lang.extensions);
+				resolvedLanguage.extensions = lang.extensions.concat(resolvedLanguage.extensions);
 			} else {
-				resolvedLanguage.extensions.push(...lang.extensions);
+				resolvedLanguage.extensions = resolvedLanguage.extensions.concat(lang.extensions);
 			}
 			for (let extension of lang.extensions) {
 				mime.registerTextMime({ id: langId, mime: primaryMime, extension: extension }, this._warnOnOverwrite);
diff --git a/src/vs/editor/test/common/services/languagesRegistry.test.ts b/src/vs/editor/test/common/services/languagesRegistry.test.ts
index 718ffd6321e..09ef74cd000 100644
--- a/src/vs/editor/test/common/services/languagesRegistry.test.ts
+++ b/src/vs/editor/test/common/services/languagesRegistry.test.ts
@@ -245,11 +245,6 @@ suite('LanguagesRegistry', () => {
 		}]);
 
 		assert.deepEqual(registry.getExtensions('a')[0], 'aExt');
-
-		registry._registerLanguages([{
-			id: 'a',
-			extensions: ['aExt2']
-		}]);
 	});
 
 	test('filenames', () => {