diff --git a/src/mac/carbon/bitmap.cpp b/src/mac/carbon/bitmap.cpp
index bb3247be56..0359fd6090 100644
--- a/src/mac/carbon/bitmap.cpp
+++ b/src/mac/carbon/bitmap.cpp
@@ -146,7 +146,7 @@ void wxBitmapRefData::Init()
     m_pictHandle = NULL ;
     m_hBitmap = NULL ;
     m_hMaskBitmap = NULL;
-    m_maskBytesPerRow = NULL ;
+    m_maskBytesPerRow = 0 ;
 
     m_rawAccessCount = 0 ;
     m_hasAlpha = false;