diff --git a/blkmaker.c b/blkmaker.c
index a5035f3..c0c1457 100644
--- a/blkmaker.c
+++ b/blkmaker.c
@@ -339,6 +339,7 @@ bool blkmk_get_mdata(blktemplate_t * const tmpl, void * const buf, const size_t
 		&& tmpl->cbtxn
 		&& blkmk_build_merkle_branches(tmpl)
 		&& bufsz >= 76
+		&& (tmpl->mutations & (BMM_CBAPPEND | BMM_CBSET))
 	))
 		return false;