diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 9ff0795..f1c1982 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -5097,6 +5097,7 @@
 requires_config_enabled MBEDTLS_RSA_C
 requires_config_enabled MBEDTLS_ECDSA_C
 run_test    "DTLS fragmenting: both (MTU)" \
+            -p "$P_PXY mtu=512" \
             "$P_SRV dtls=1 debug_level=2 auth_mode=required \
              crt_file=data_files/server7_int-ca.crt \
              key_file=data_files/server7.key \