diff --git a/agent/command-ssh.c b/agent/command-ssh.c
index 689241f67..88686200a 100644
--- a/agent/command-ssh.c
+++ b/agent/command-ssh.c
@@ -1964,7 +1964,7 @@ ssh_key_to_blob (gcry_sexp_t sexp, int with_secret,
               err = gpg_error (GPG_ERR_INV_SEXP);
               goto out;
             }
-          if (*p_elems == 'q')
+          if (*p_elems == 'q' && datalen)
             { /* Remove the prefix 0x40.  */
               data++;
               datalen--;