diff --git a/Jakefile.js b/Jakefile.js
index b3e18e8cb1a..ad853238111 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -143,6 +143,7 @@ var harnessSources = harnessCoreSources.concat([
     "customTransforms.ts",
     "programMissingFiles.ts",
     "symbolWalker.ts",
+    "languageService.ts",
 ].map(function (f) {
     return path.join(unittestsDirectory, f);
 })).concat([
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 2a07d2b6560..725070c02bf 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -32,7 +32,6 @@ namespace ts {
     }
 
     const stringWriter = createSingleLineStringWriter();
-    let stringWriterAcquired = false;
 
     function createSingleLineStringWriter(): StringSymbolWriter {
         let str = "";
@@ -62,15 +61,14 @@ namespace ts {
     }
 
     export function usingSingleLineStringWriter(action: (writer: StringSymbolWriter) => void): string {
+        const oldString = stringWriter.string();
         try {
-            Debug.assert(!stringWriterAcquired);
-            stringWriterAcquired = true;
             action(stringWriter);
             return stringWriter.string();
         }
         finally {
             stringWriter.clear();
-            stringWriterAcquired = false;
+            stringWriter.writeKeyword(oldString);
         }
     }
 
diff --git a/src/harness/tsconfig.json b/src/harness/tsconfig.json
index 1469079bcaa..bd7c9bc2ffa 100644
--- a/src/harness/tsconfig.json
+++ b/src/harness/tsconfig.json
@@ -128,6 +128,7 @@
         "./unittests/extractMethods.ts",
         "./unittests/textChanges.ts",
         "./unittests/telemetry.ts",
+        "./unittests/languageService.ts",
         "./unittests/programMissingFiles.ts"
     ]
 }
diff --git a/src/harness/unittests/languageService.ts b/src/harness/unittests/languageService.ts
new file mode 100644
index 00000000000..9c838845c20
--- /dev/null
+++ b/src/harness/unittests/languageService.ts
@@ -0,0 +1,49 @@
+/// <reference path="..\harness.ts" />
+
+namespace ts {
+    describe("languageService", () => {
+        const files: {[index: string]: string} = {
+            "foo.ts": `import Vue from "./vue";
+import Component from "./vue-class-component";
+import { vueTemplateHtml } from "./variables";
+
+@Component({
+    template: vueTemplateHtml,
+})
+class Carousel<T> extends Vue {
+}`,
+            "variables.ts": `export const vueTemplateHtml = \`<div></div>\`;`,
+            "vue.d.ts": `export namespace Vue { export type Config = { template: string }; }`,
+            "vue-class-component.d.ts": `import Vue from "./vue";
+export function Component(x: Config): any;`
+};
+        it("should be able to create a language service which can respond to deinition requests without throwing", () => {
+            const languageService = ts.createLanguageService({
+                getCompilationSettings() {
+                    return {};
+                },
+                getScriptFileNames() {
+                    return ["foo.ts", "variables.ts", "vue.d.ts", "vue-class-component.d.ts"];
+                },
+                getScriptVersion(_fileName) {
+                    return "";
+                },
+                getScriptSnapshot(fileName) {
+                    if (fileName === ".ts") {
+                        return ts.ScriptSnapshot.fromString("");
+                    }
+                    return ts.ScriptSnapshot.fromString(files[fileName] || "");
+                },
+                getCurrentDirectory: () => ".",
+                getDefaultLibFileName(options) {
+                    return ts.getDefaultLibFilePath(options);
+                },
+                fileExists: noop as any,
+                readFile: noop as any,
+                readDirectory: noop as any,
+            });
+            const definitions = languageService.getDefinitionAtPosition("foo.ts", 160); // 160 is the latter `vueTemplateHtml` position
+            expect(definitions).to.exist;
+        });
+    });
+}
\ No newline at end of file