diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index e258cf8a8f7..2ed88a6e456 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -827,8 +827,8 @@ namespace Harness {
             return libFileNameSourceFileMap[fileName];
         }
 
-        export function getDefaultLibSourceFile(languageVersion: ts.ScriptTarget) {
-            return languageVersion === ts.ScriptTarget.ES6 ? getDefaultLibrarySourceFile(es2015DefaultLibFileName) : getDefaultLibrarySourceFile(defaultLibFileName);
+        export function getDefaultLibFileName(options: ts.CompilerOptions): string {
+            return options.target === ts.ScriptTarget.ES6 ? es2015DefaultLibFileName : defaultLibFileName
         }
 
         // Cache these between executions so we don't have to re-parse them for every test
@@ -887,7 +887,7 @@ namespace Harness {
             return {
                 getCurrentDirectory: () => currentDirectory,
                 getSourceFile,
-                getDefaultLibFileName: options => options.target === ts.ScriptTarget.ES6 ? es2015DefaultLibFileName : defaultLibFileName,
+                getDefaultLibFileName,
                 writeFile,
                 getCanonicalFileName,
                 useCaseSensitiveFileNames: () => useCaseSensitiveFileNames,
diff --git a/src/harness/projectsRunner.ts b/src/harness/projectsRunner.ts
index 452ce5dbff3..6db745bd7a0 100644
--- a/src/harness/projectsRunner.ts
+++ b/src/harness/projectsRunner.ts
@@ -156,7 +156,7 @@ class ProjectRunner extends RunnerBase {
             function getSourceFile(fileName: string, languageVersion: ts.ScriptTarget): ts.SourceFile {
                 let sourceFile: ts.SourceFile = undefined;
                 if (fileName === Harness.Compiler.defaultLibFileName) {
-                    sourceFile = Harness.Compiler.getDefaultLibSourceFile(languageVersion);
+                    sourceFile = Harness.Compiler.getDefaultLibrarySourceFile(Harness.Compiler.getDefaultLibFileName(compilerOptions));
                 }
                 else {
                     const text = getSourceFileText(fileName);