keep the slice.step field as NULL if no step expression is given
diff --git a/Misc/NEWS b/Misc/NEWS
index 3a4e1ac..1192618 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- In the slice AST type, the step field will always be None if a step expression
+  is not specified.
+
 - Issue #4547: When debugging a very large function, it was not always
   possible to update the lineno attribute of the current frame.