diff --git a/src/Graphics/Graphics.cpp b/src/Graphics/Graphics.cpp
index d12f18d..91aef50 100644
--- a/src/Graphics/Graphics.cpp
+++ b/src/Graphics/Graphics.cpp
@@ -40,6 +40,7 @@ struct GrafPortImpl
 		macpm = {};
 		macpm.bounds = boundsRect;
 		macpm.pixelSize = 32;
+		macpm.rowBytes = (pixels.width * macpm.pixelSize / 8) | (1 << 15);		// bit 15 = 1: structure is PixMap, not BitMap
 		macpm._impl = (Ptr) &pixels;
 		macpmPtr = &macpm;
 	}
diff --git a/src/PommeTypes.h b/src/PommeTypes.h
index 059e761..634e5c5 100644
--- a/src/PommeTypes.h
+++ b/src/PommeTypes.h
@@ -143,6 +143,7 @@ typedef struct PixMap
 {
 	Rect bounds;
 	short pixelSize;
+	short rowBytes;
 	Ptr _impl;		// Points to ARGBPixmap
 } PixMap;
 typedef PixMap*							PixMapPtr;