diff --git a/lib/cmd/cov.js b/lib/cmd/cov.js
index 489fe822..7fc73548 100644
--- a/lib/cmd/cov.js
+++ b/lib/cmd/cov.js
@@ -41,11 +41,6 @@ class CovCommand extends Command {
     process.env.NODE_ENV = 'test';
     process.env.TMPDIR = tmpDir;
 
-    const opt = {
-      env: Object.assign({}, process.env),
-      execArgv,
-    };
-
     // istanbul coverage ignore
     const excludes = argv.x || (process.env.COV_EXCLUDES && process.env.COV_EXCLUDES.split(',')) || [];
     for (const exclude of excludes) {
@@ -54,12 +49,17 @@ class CovCommand extends Command {
     argv.x = undefined;
 
     const covFile = require.resolve('istanbul/lib/cli.js');
-    // resolve istanbul path for coffee
-    /* istanbul ignore next */
-    process.env.istanbul_bin_path = covFile;
     const coverageDir = path.join(cwd, 'coverage');
     yield rimraf(coverageDir);
 
+    const opt = {
+      execArgv,
+      // resolve istanbul path for coffee
+      env: Object.assign({
+        istanbul_bin_path: covFile,
+      }, process.env),
+    };
+
     // save coverage-xxxx.json to $PWD/coverage
     const covArgs = this.getCovArgs(argv);
     debug('covArgs: %j', covArgs);
diff --git a/package.json b/package.json
index 245b3a60..2069d067 100644
--- a/package.json
+++ b/package.json
@@ -18,6 +18,7 @@
     "glob": "^7.1.1",
     "globby": "^6.1.0",
     "intelli-espower-loader": "^1.0.1",
+    "is-type-of": "^1.0.0",
     "istanbul": "^1.1.0-alpha.1",
     "mocha": "^3.2.0",
     "mz-modules": "^1.0.0",