diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index a0fb88213ed..a3789eb9ebf 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -844,9 +844,7 @@ namespace Harness {
         export const es2015DefaultLibFileName = "lib.es2015.d.ts";
 
         // Cache of lib files from "built/local"
-        const libFileNameSourceFileMap = ts.createMapFromTemplate<ts.SourceFile>({
-            [defaultLibFileName]: createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + "lib.es5.d.ts"), /*languageVersion*/ ts.ScriptTarget.Latest)
-        });
+        let libFileNameSourceFileMap: ts.Map<ts.SourceFile> | undefined;
 
         // Cache of lib files from  "tests/lib/"
         const testLibFileNameSourceFileMap = ts.createMap<ts.SourceFile>();
@@ -857,6 +855,12 @@ namespace Harness {
                 return undefined;
             }
 
+            if (!libFileNameSourceFileMap) {
+                libFileNameSourceFileMap = ts.createMapFromTemplate({
+                    [defaultLibFileName]: createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + "lib.es5.d.ts"), /*languageVersion*/ ts.ScriptTarget.Latest)
+                });
+            }
+
             let sourceFile = libFileNameSourceFileMap.get(fileName);
             if (!sourceFile) {
                 libFileNameSourceFileMap.set(fileName, sourceFile = createSourceFileAndAssertInvariants(fileName, IO.readFile(libFolder + fileName), ts.ScriptTarget.Latest));
diff --git a/src/harness/unittests/moduleResolution.ts b/src/harness/unittests/moduleResolution.ts
index 0acbe9450bb..32301d6dccf 100644
--- a/src/harness/unittests/moduleResolution.ts
+++ b/src/harness/unittests/moduleResolution.ts
@@ -392,7 +392,7 @@ export = C;
     });
 
     describe("Files with different casing", () => {
-        const library = createSourceFile("lib.d.ts", "", ScriptTarget.ES5);
+        let library: SourceFile;
         function test(files: Map<string>, options: CompilerOptions, currentDirectory: string, useCaseSensitiveFileNames: boolean, rootFiles: string[], diagnosticCodes: number[]): void {
             const getCanonicalFileName = createGetCanonicalFileName(useCaseSensitiveFileNames);
             if (!useCaseSensitiveFileNames) {
@@ -406,6 +406,9 @@ export = C;
             const host: CompilerHost = {
                 getSourceFile: (fileName: string, languageVersion: ScriptTarget) => {
                     if (fileName === "lib.d.ts") {
+                        if (!library) {
+                            library = createSourceFile("lib.d.ts", "", ScriptTarget.ES5);
+                        }
                         return library;
                     }
                     const path = getCanonicalFileName(normalizePath(combinePaths(currentDirectory, fileName)));