--- a/editors/SFCViewer.py	Tue Aug 15 16:48:49 2017 +0300
+++ b/editors/SFCViewer.py	Tue Aug 15 17:01:51 2017 +0300
@@ -342,7 +342,7 @@
 
     # This method check the IEC 61131-3 compatibility between two SFC blocks
     def BlockCompatibility(self, startblock = None, endblock = None, direction = None):
-        if startblock != None and endblock != None and (isinstance(startblock, SFC_Objects)\
+        if startblock is not None and endblock is not None and (isinstance(startblock, SFC_Objects)\
                                                                or isinstance(endblock, SFC_Objects)):
             # Full "SFC_StandardRules" table would be symmetrical and
             # to avoid duplicate records and minimize the table only upper part is defined.