diff --git a/src/richtext/richtextbuffer.cpp b/src/richtext/richtextbuffer.cpp
index 6223e5b950..24859551f1 100644
--- a/src/richtext/richtextbuffer.cpp
+++ b/src/richtext/richtextbuffer.cpp
@@ -36,6 +36,7 @@
 #include "wx/textfile.h"
 #include "wx/hashmap.h"
 #include "wx/dynarray.h"
+#include "wx/math.h"
 
 #include "wx/richtext/richtextctrl.h"
 #include "wx/richtext/richtextstyles.h"
@@ -709,7 +710,9 @@ bool wxRichTextObject::DrawBoxAttributes(wxDC& dc, wxRichTextBuffer* buffer, con
             // Let's pretend our background is always white. Calculate a colour value
             // from this and the opacity.
             double p = attr.GetTextBoxAttr().GetShadow().GetOpacity().GetValue() / 100.0;
-            shadowColour.Set((1.0 - p)*255 + p*shadowColour.Red(), (1.0 - p)*255 + p*shadowColour.Green(), (1.0 - p)*255 + p*shadowColour.Blue());
+            shadowColour.Set(wxRound((1.0 - p)*255 + p*shadowColour.Red()),
+                             wxRound((1.0 - p)*255 + p*shadowColour.Green()),
+                             wxRound((1.0 - p)*255 + p*shadowColour.Blue()));
         }
         wxRect shadowRect = borderRect;
         if (attr.GetTextBoxAttr().GetShadow().GetOffsetX().IsValid())