diff --git a/crypto/vmac.c b/crypto/vmac.c
index df76a81..3034454 100644
--- a/crypto/vmac.c
+++ b/crypto/vmac.c
@@ -655,6 +655,10 @@
 	if (IS_ERR(alg))
 		return PTR_ERR(alg);
 
+	err = -EINVAL;
+	if (alg->cra_blocksize != 16)
+		goto out_put_alg;
+
 	inst = shash_alloc_instance("vmac", alg);
 	err = PTR_ERR(inst);
 	if (IS_ERR(inst))