diff --git a/range.cpp b/range.cpp
index 3e70aa82..284214ac 100644
--- a/range.cpp
+++ b/range.cpp
@@ -90,6 +90,7 @@ range<T>::expand (T val) {
 template <typename T>
 double
 range<T>::normalise (T val) const {
+    CHECK_SOFT (val >= min && val <= max);
     return ((double)val - min) /
            ((double)max - min);
 }