diff --git a/src/tools/pkcs11-tool.c b/src/tools/pkcs11-tool.c
index 4d037a5b..4caa4abe 100644
--- a/src/tools/pkcs11-tool.c
+++ b/src/tools/pkcs11-tool.c
@@ -163,7 +163,7 @@ struct x509cert_info {
 	int		subject_len;
 	unsigned char	issuer[256];
 	int		issuer_len;
-	unsigned char	serialnum[256];
+	unsigned char	serialnum[128];
 	int		serialnum_len;
 };
 struct rsakey_info {