diff --git a/easyprocess/__init__.py b/easyprocess/__init__.py
index <HASH>..<HASH> 100644
--- a/easyprocess/__init__.py
+++ b/easyprocess/__init__.py
@@ -160,7 +160,7 @@ class EasyProcess(object):
         ok = ret == return_code
         if not ok:
             raise EasyProcessError(
-                self, 'check error, return code is not {}!'.format(return_code))
+                self, 'check error, return code is not {0}!'.format(return_code))
         return self
 
     def check_installed(self):
Update __init__.py

fix format() field modifier to be backward-compatible with Python<I> (detected by Travis CI)