diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index e02bb25e094..2abcee8c6d5 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -292,7 +292,7 @@ namespace FourSlash {
                 // Check if no-default-lib flag is false and if so add default library
                 if (!resolvedResult.isLibFile) {
                     this.languageServiceAdapterHost.addScript(Harness.Compiler.defaultLibFileName,
-                        Harness.Compiler.getDefaultLibrarySourceFile(Harness.Compiler.defaultLibFileName).text);
+                        Harness.Compiler.getDefaultLibrarySourceFile().text);
                 }
             }
             else {
@@ -303,7 +303,7 @@ namespace FourSlash {
                     }
                 });
                 this.languageServiceAdapterHost.addScript(Harness.Compiler.defaultLibFileName,
-                    Harness.Compiler.getDefaultLibrarySourceFile(Harness.Compiler.defaultLibFileName).text);
+                    Harness.Compiler.getDefaultLibrarySourceFile().text);
             }
 
             this.formatCodeOptions = {
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index b4be499e141..a86d77c22c9 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -815,8 +815,9 @@ namespace Harness {
             [defaultLibFileName]: createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + "lib.es5.d.ts"), /*languageVersion*/ ts.ScriptTarget.Latest)
         };
 
-        export function getDefaultLibrarySourceFile(fileName: string): ts.SourceFile {
+        export function getDefaultLibrarySourceFile(fileName = defaultLibFileName): ts.SourceFile {
             if (!isLibraryFile(fileName)) {
+                assert(!isLibraryFile(fileName), "Expected library fileName");
                 return undefined;
             }
 
@@ -827,7 +828,7 @@ namespace Harness {
         }
 
         export function getDefaultLibSourceFile(languageVersion: ts.ScriptTarget) {
-            return languageVersion === ts.ScriptTarget.ES6 ? getDefaultLibrarySourceFile("lib.es2015.d.ts") : getDefaultLibrarySourceFile(defaultLibFileName);
+            return languageVersion === ts.ScriptTarget.ES6 ? getDefaultLibrarySourceFile(es2015DefaultLibFileName) : getDefaultLibrarySourceFile(defaultLibFileName);
         }
 
         // Cache these between executions so we don't have to re-parse them for every test