diff --git a/clamp.py b/clamp.py
index d6b5ded..253bb06 100644
--- a/clamp.py
+++ b/clamp.py
@@ -1,2 +1,4 @@
 def clamp(value, lower, upper):
-    return min(value, upper)
+    if lower > upper:
+        raise ValueError("lower must not be greater than upper")
+    return max(lower, min(value, upper))