diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index ba7d8ca9ce3..be6d083e3a5 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -422,10 +422,13 @@ namespace ts {
                 if (json["files"] instanceof Array) {
                     fileNames = map(<string[]>json["files"], s => combinePaths(basePath, s));
                 }
+                else {
+                    errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, 'files', 'Array'));                    
+                }
             }
             else {
                 let exclude = json["exclude"] instanceof Array ? map(<string[]>json["exclude"], normalizeSlashes) : undefined;
-                let sysFiles = host.readDirectory(basePath, ".ts", exclude).concat(host.readDirectory(basePath, ".tsx", exclude));
+                let sysFiles = host.readDirectory(basePath, ".ts", exclude).concat(host.readDirectory(basePath, ".tsx", exclude));
                 for (let i = 0; i < sysFiles.length; i++) {
                     let name = sysFiles[i];
                     if (fileExtensionIs(name, ".d.ts")) {