diff --git a/wizard-work/beam/clamp.py b/wizard-work/beam/clamp.py
index 801a8e2..a66b7e9 100644
--- a/wizard-work/beam/clamp.py
+++ b/wizard-work/beam/clamp.py
@@ -3,6 +3,8 @@ from __future__ import annotations
 
 
 def clamp(value, lo, hi):
+    if value is None or lo is None or hi is None:
+        raise ValueError("None is not a clampable value: value=%r, lo=%r, hi=%r" % (value, lo, hi))
     if lo > hi:
         raise ValueError("lo > hi: %r > %r" % (lo, hi))
     if value < lo: