diff --git a/src/gui/intro.cpp b/src/gui/intro.cpp
index 620f2f25e..e768427bc 100644
--- a/src/gui/intro.cpp
+++ b/src/gui/intro.cpp
@@ -17,6 +17,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#define _USE_MATH_DEFINES
 #include "gui.h"
 #include "imgui_internal.h"