diff --git a/src/Range.cpp b/src/Range.cpp
index 69aca85d..5eaee6c6 100644
--- a/src/Range.cpp
+++ b/src/Range.cpp
@@ -27,6 +27,7 @@
 #include <cmake.h>
 #include <Range.h>
 #include <sstream>
+#include <cassert>
 
 ////////////////////////////////////////////////////////////////////////////////
 // A Range consists of a start time and optional end time. A missing end
@@ -285,6 +286,8 @@ std::vector <Range> Range::subtract (const Range& other) const
 // If the range is open, use 'now' as the end.
 time_t Range::total () const
 {
+  assert (end >= start);
+
   if (is_ended ())
     return Datetime (end) - Datetime (start);