diff --git a/indra/llwindow/llmousehandler.h b/indra/llwindow/llmousehandler.h
index bd163535a0b0fc01cffb52842722c5096c5e9295..cdde2ce0e01d854bb0ed7faeaed661f3bf07946f 100644
--- a/indra/llwindow/llmousehandler.h
+++ b/indra/llwindow/llmousehandler.h
@@ -13,6 +13,8 @@
 // Intended for use via multiple inheritance. 
 // A class may have as many interfaces as it likes, but never needs to inherit one more than once.
 
+#include "llstring.h"
+
 class LLMouseHandler
 {
 public: