diff --git a/src/GUI/UnitsModel.cxx b/src/GUI/UnitsModel.cxx
index 0f81e72d8..525babcd2 100644
--- a/src/GUI/UnitsModel.cxx
+++ b/src/GUI/UnitsModel.cxx
@@ -21,6 +21,7 @@
 #include "UnitsModel.hxx"
 
 #include <cmath>
+#include <cassert>
 
 #include <simgear/constants.h>