diff --git a/src/Mod/Import/stepZ.py b/src/Mod/Import/stepZ.py
index 89c134180f..96fe1ea9ae 100644
--- a/src/Mod/Import/stepZ.py
+++ b/src/Mod/Import/stepZ.py
@@ -33,7 +33,7 @@ else:  # six.PY2
 
 def mkz_string(input):
     if six.PY3:
-        if not isinstance(input, str):
+        if isinstance(input, str):
             return input
         else:
             input = input.encode('utf-8')