diff --git a/Magick++/lib/STL.cpp b/Magick++/lib/STL.cpp
index e6fe6fc..e17b766 100644
--- a/Magick++/lib/STL.cpp
+++ b/Magick++/lib/STL.cpp
@@ -678,7 +678,7 @@
 
 // Floodfill designated area with a matte value
 Magick::matteFloodfillImage::matteFloodfillImage( const Color &target_ ,
-                                                  const size_t matte_,
+                                                  const unsigned int matte_,
                                                   const ssize_t x_, const ssize_t y_,
                                                   const PaintMethod method_ )
   : _target( target_ ),