diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 561cf2c8b0..6a55948815 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -810,15 +810,16 @@ module Harness {
         }
 
         export function createSourceFileAndAssertInvariants(
-            fileName: string,
-            sourceText: string,
-            languageVersion: ts.ScriptTarget,
-            assertInvariants: boolean) {
-
+                fileName: string,
+                sourceText: string,
+                languageVersion: ts.ScriptTarget) {
+            // We'll only assert invariants outside of light mode. 
+            const shouldAssertInvariants = !Harness.lightMode;
+            
             // Only set the parent nodes if we're asserting invariants.  We don't need them otherwise.
-            var result = ts.createSourceFile(fileName, sourceText, languageVersion, /*setParentNodes:*/ assertInvariants);
+            var result = ts.createSourceFile(fileName, sourceText, languageVersion, /*setParentNodes:*/ shouldAssertInvariants);
 
-            if (assertInvariants) {
+            if (shouldAssertInvariants) {
                 Utils.assertInvariants(result, /*parent:*/ undefined);
             }
 
@@ -829,8 +830,8 @@ module Harness {
         const lineFeed = "\n";
 
         export var defaultLibFileName = 'lib.d.ts';
-        export var defaultLibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest, /*assertInvariants*/ !Harness.lightMode);
-        export var defaultES6LibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.es6.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest, /*assertInvariants*/ !Harness.lightMode);
+        export var defaultLibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest);
+        export var defaultES6LibSourceFile = createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + 'lib.core.es6.d.ts'), /*languageVersion*/ ts.ScriptTarget.Latest);
 
         // Cache these between executions so we don't have to re-parse them for every test
         export var fourslashFileName = 'fourslash.ts';
@@ -861,7 +862,7 @@ module Harness {
             function register(file: { unitName: string; content: string; }) {
                 if (file.content !== undefined) {
                     var fileName = ts.normalizePath(file.unitName);
-                    filemap[getCanonicalFileName(fileName)] = createSourceFileAndAssertInvariants(fileName, file.content, scriptTarget, /*assertInvariants*/ !Harness.lightMode);
+                    filemap[getCanonicalFileName(fileName)] = createSourceFileAndAssertInvariants(fileName, file.content, scriptTarget);
                 }
             };
             inputFiles.forEach(register);
@@ -884,7 +885,7 @@ module Harness {
                     }
                     else if (fn === fourslashFileName) {
                         var tsFn = 'tests/cases/fourslash/' + fourslashFileName;
-                        fourslashSourceFile = fourslashSourceFile || createSourceFileAndAssertInvariants(tsFn, Harness.IO.readFile(tsFn), scriptTarget, /*assertInvariants*/ !Harness.lightMode);
+                        fourslashSourceFile = fourslashSourceFile || createSourceFileAndAssertInvariants(tsFn, Harness.IO.readFile(tsFn), scriptTarget);
                         return fourslashSourceFile;
                     }
                     else {
diff --git a/src/harness/projectsRunner.ts b/src/harness/projectsRunner.ts
index bcff2ef193..324524272e 100644
--- a/src/harness/projectsRunner.ts
+++ b/src/harness/projectsRunner.ts
@@ -174,7 +174,7 @@ class ProjectRunner extends RunnerBase {
                 else {
                     var text = getSourceFileText(fileName);
                     if (text !== undefined) {
-                        sourceFile = Harness.Compiler.createSourceFileAndAssertInvariants(fileName, text, languageVersion, /*assertInvariants*/ !Harness.lightMode);
+                        sourceFile = Harness.Compiler.createSourceFileAndAssertInvariants(fileName, text, languageVersion);
                     }
                 }