diff --git a/lib/tfw/components/webide_event_handler.py b/lib/tfw/components/webide_event_handler.py
index 0a303ed..abe9e21 100644
--- a/lib/tfw/components/webide_event_handler.py
+++ b/lib/tfw/components/webide_event_handler.py
@@ -58,7 +58,7 @@ class FileManager:  # pylint: disable=too-many-instance-attributes
 
     @filename.setter
     def filename(self, filename):
-        if not filename in self.files:
+        if filename not in self.files:
             raise EnvironmentError('No such file in workdir!')
         self._filename = filename