diff --git a/magick/blob-private.h b/magick/blob-private.h
index 409605e..4384b0a 100644
--- a/magick/blob-private.h
+++ b/magick/blob-private.h
@@ -79,7 +79,7 @@
 
 extern MagickExport  MagickBooleanType
   CloseBlob(Image *),
-  DiscardBlobBytes(Image *,const size_t),
+  DiscardBlobBytes(Image *,const MagickSizeType),
   OpenBlob(const ImageInfo *,Image *,const BlobMode,ExceptionInfo *),
   SetBlobExtent(Image *,const MagickSizeType),
   UnmapBlob(void *,const size_t);