diff --git a/fb/fb.h b/fb/fb.h
index 22ed4f78a..e32c10bfb 100644
--- a/fb/fb.h
+++ b/fb/fb.h
@@ -601,7 +601,7 @@ extern WindowPtr    *WindowTable;
 
 /* Framebuffer access wrapper */
 #ifdef FB_ACCESS_WRAPPER
-typedef FbBits (*ReadMemoryProcPtr)(void *src, int size);
+typedef FbBits (*ReadMemoryProcPtr)(const void *src, int size);
 typedef void (*WriteMemoryProcPtr)(void *dst, FbBits value, int size);
 typedef void (*SetupWrapProcPtr)(ReadMemoryProcPtr  *pRead,
                                  WriteMemoryProcPtr *pWrite,