diff --git a/modules/mac-parts.c b/modules/mac-parts.c
index d7c0c05..43e98b6 100644
--- a/modules/mac-parts.c
+++ b/modules/mac-parts.c
@@ -74,6 +74,7 @@ macparts_open( macparts_info_t *di )
 			bs = 512;
 	}
 	if (parnum == 0) {
+		di->blocksize =(uint)bs;
 		di->offs = (llong)0;
 		di->size = (llong)dmap.sbBlkCount * bs;
 		PUSH( -1 );