diff --git a/src/auth.c b/src/auth.c
index 8a686dc2..1baec109 100644
--- a/src/auth.c
+++ b/src/auth.c
@@ -707,6 +707,7 @@ static int ssh_userauth_agent_publickey(ssh_session session,
 
     rc = ssh_buffer_add_ssh_string(session->out_buffer, str);
     ssh_string_free(str);
+    str = NULL;
     if (rc < 0) {
         goto fail;
     }