diff --git a/nutree/common.py b/nutree/common.py
index 9d57ed4..4face0c 100644
--- a/nutree/common.py
+++ b/nutree/common.py
@@ -297,7 +297,10 @@ def call_traversal_cb(fn: Callable, node: Node, memo: Any) -> False | None:
     """
     try:
         res = fn(node, memo)
-        if res is SkipBranch or isinstance(res, SkipBranch):
+
+        if res is None:
+            return None
+        elif res is SkipBranch or isinstance(res, SkipBranch):
             return False
         elif res is StopTraversal or isinstance(res, StopTraversal):
             raise res
@@ -306,10 +309,10 @@ def call_traversal_cb(fn: Callable, node: Node, memo: Any) -> False | None:
         elif res is StopIteration or isinstance(res, StopIteration):
             # Converts wrong syntax in exception handler...
             raise res
-        elif res is not None:
+        else:
             raise ValueError(
                 "callback should not return values except for "
-                f"False, SkipBranch, or StopTraversal: {res!r}."
+                f"None, False, SkipBranch, or StopTraversal: {res!r}."
             )
     except SkipBranch:
         return False