diff --git a/src/Main/viewmgr.hxx b/src/Main/viewmgr.hxx
index c6476a15b..8053a1a9d 100644
--- a/src/Main/viewmgr.hxx
+++ b/src/Main/viewmgr.hxx
@@ -30,6 +30,8 @@
 #include <simgear/structure/subsystem_mgr.hxx>
 #include <simgear/math/SGMath.hxx>
 
+#include <Main/viewer.hxx> // FIXME - remove inlines here.
+
 // forward decls
 class FGViewer;
 typedef SGSharedPtr<FGViewer> FGViewerPtr;