diff --git a/include/wx/motif/bitmap.h b/include/wx/motif/bitmap.h
index 6cc1da1765..76f3912dd2 100644
--- a/include/wx/motif/bitmap.h
+++ b/include/wx/motif/bitmap.h
@@ -205,7 +205,7 @@ protected:
 
 // Creates a bitmap with transparent areas drawn in
 // the given colour.
-wxBitmap wxCreateMaskedBitmap(wxBitmap& bitmap, wxColour& colour);
+wxBitmap wxCreateMaskedBitmap(const wxBitmap& bitmap, wxColour& colour);
 
 #endif
   // _WX_BITMAP_H_
diff --git a/src/motif/bitmap.cpp b/src/motif/bitmap.cpp
index 443000b14b..c7fb9ee8e8 100644
--- a/src/motif/bitmap.cpp
+++ b/src/motif/bitmap.cpp
@@ -1022,7 +1022,7 @@ XCreateInsensitivePixmap( Display *display, Pixmap pixmap )
 
 // Creates a bitmap with transparent areas drawn in
 // the given colour.
-wxBitmap wxCreateMaskedBitmap(wxBitmap& bitmap, wxColour& colour)
+wxBitmap wxCreateMaskedBitmap(const wxBitmap& bitmap, wxColour& colour)
 {
     wxBitmap newBitmap(bitmap.GetWidth(),
         bitmap.GetHeight(),