diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index c56a18b3ab2..d22bc986688 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -379,10 +379,10 @@ namespace ts {
       * Read tsconfig.json file
       * @param fileName The path to the config file
       */
-    export function readConfigFile(fileName: string): { config?: any; error?: Diagnostic }  {
+    export function readConfigFile(fileName: string, readFile: (path: string) => string): { config?: any; error?: Diagnostic }  {
         let text = "";
         try {
-            text = sys.readFile(fileName);
+            text = readFile(fileName);
         }
         catch (e) {
             return { error: createCompilerDiagnostic(Diagnostics.Cannot_read_file_0_Colon_1, fileName, e.message) };
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index 8c0a6d4e8c4..96759b68250 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -216,7 +216,7 @@ namespace ts {
             if (!cachedProgram) {
                 if (configFileName) {
 
-                    let result = readConfigFile(configFileName);
+                    let result = readConfigFile(configFileName, sys.readFile);
                     if (result.error) {
                         reportDiagnostic(result.error);
                         return sys.exit(ExitStatus.DiagnosticsPresent_OutputsSkipped);