diff --git a/h2o-py/h2o/frame.py b/h2o-py/h2o/frame.py
index <HASH>..<HASH> 100644
--- a/h2o-py/h2o/frame.py
+++ b/h2o-py/h2o/frame.py
@@ -664,7 +664,7 @@ class H2OVec:
       # whole vec replacement
       self._len_check(b)
       # lazy update in-place of the whole vec
-      self._expr = Expr("=", Expr("[", self._expr, b), Expr(c))
+      self._expr = Expr("=", Expr("[", self._expr, b), None if c is None else Expr(c))
     else:
       raise NotImplementedError("Only vector replacement is currently supported.")
handle assign null over 0

... as well as assign 0 over null