diff --git a/rtgui/main.cc b/rtgui/main.cc
index f268139f8..975d1ef32 100644
--- a/rtgui/main.cc
+++ b/rtgui/main.cc
@@ -22,6 +22,12 @@
 // newer (non customized) versions of this file go to raw.cc_new
 
 // This file is for your program, I won't touch it again!
+
+#ifdef __GNUC__
+	#if defined(__FAST_MATH__)
+		#error Using the -ffast-math CFLAG is known to lead to problems. Disable it to compile RawTherapee.
+	#endif
+#endif
 
 #include "config.h"
 #include <gtkmm.h>