diff --git a/programs/pkey/ecdsa.c b/programs/pkey/ecdsa.c
index be2ca90..953c144 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -73,7 +73,7 @@
     size_t len;
 
     if (mbedtls_ecp_point_write_binary(&key->MBEDTLS_PRIVATE(grp), &key->MBEDTLS_PRIVATE(Q),
-                                       MBEDTLS_ECP_PF_UNCOMPRESSED, &len, buf, sizeof buf) != 0) {
+                                       MBEDTLS_ECP_PF_UNCOMPRESSED, &len, buf, sizeof(buf)) != 0) {
         mbedtls_printf("internal error\n");
         return;
     }