allow global Promise
diff --git a/src/exec.js b/src/exec.js
index d3f2e44..2ab268d 100644
--- a/src/exec.js
+++ b/src/exec.js
@@ -17,6 +17,8 @@
  under the License.
  */
 
+/* globals Promise: true */
+
 var child_process = require('child_process');
 
 /**