diff --git a/programs/pkey/ecdh_curve25519.c b/programs/pkey/ecdh_curve25519.c
index d880a1a..9804417 100644
--- a/programs/pkey/ecdh_curve25519.c
+++ b/programs/pkey/ecdh_curve25519.c
@@ -74,7 +74,7 @@
     if ((ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func,
                                      &entropy,
                                      (const unsigned char *) pers,
-                                     sizeof pers)) != 0) {
+                                     sizeof(pers))) != 0) {
         mbedtls_printf(" failed\n  ! mbedtls_ctr_drbg_seed returned %d\n",
                        ret);
         goto exit;