diff --git a/src/servers/app/GlyphLayoutEngine.h b/src/servers/app/GlyphLayoutEngine.h
index 36ddfe23e5..26690ef83c 100644
--- a/src/servers/app/GlyphLayoutEngine.h
+++ b/src/servers/app/GlyphLayoutEngine.h
@@ -15,6 +15,7 @@
 #include "FontCacheEntry.h"
 #include "ServerFont.h"
 
+#include <ctype.h>
 
 class FontCacheReference {
  public: