diff --git a/tests/sys/net/if_bridge_test.sh b/tests/sys/net/if_bridge_test.sh
index da797f4fd39a..3c0a31ca1076 100755
--- a/tests/sys/net/if_bridge_test.sh
+++ b/tests/sys/net/if_bridge_test.sh
@@ -577,6 +577,11 @@ mtu_body()
 	check_mtu ${bridge} 2000
 	check_mtu ${gif} 2000
 	check_mtu ${epair}a 2000
+
+	# We're not allowed to change the MTU of a member interface
+	atf_check -s exit:1 -e ignore \
+		ifconfig ${epair}a mtu 1900
+	check_mtu ${epair}a 2000
 }
 
 mtu_cleanup()