diff --git a/simgear/structure/SGExpression.cxx b/simgear/structure/SGExpression.cxx
index 6f292e46..df0aa9d7 100644
--- a/simgear/structure/SGExpression.cxx
+++ b/simgear/structure/SGExpression.cxx
@@ -27,7 +27,6 @@
 #include "Singleton.hxx"
 
 #include <algorithm>
-#include <functional>
 #include <map>
 #include <utility>
 #include <string>
diff --git a/simgear/structure/SGExpression.hxx b/simgear/structure/SGExpression.hxx
index 420a9045..a6012a0d 100644
--- a/simgear/structure/SGExpression.hxx
+++ b/simgear/structure/SGExpression.hxx
@@ -24,6 +24,7 @@
 
 #include <string>
 #include <vector>
+#include <functional>
 
 #include <simgear/props/condition.hxx>
 #include <simgear/props/props.hxx>