diff --git a/tox.ini b/tox.ini
index 947ec38..270aea9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,8 @@
 envlist = py27,py3
 [pep8]
 max-line-length = 100
+[pep257]
+add-ignore = D401
 [testenv]
 deps=
     pytest
diff --git a/trezor_agent/device/interface.py b/trezor_agent/device/interface.py
index 32bd44e..14bd9ad 100644
--- a/trezor_agent/device/interface.py
+++ b/trezor_agent/device/interface.py
@@ -54,7 +54,7 @@ class NotFoundError(Error):
 
 
 class DeviceError(Error):
-    """"Error during device operation."""
+    """Error during device operation."""
 
 
 class Identity(object):