diff --git a/bin/tests/system/isctest/check.py b/bin/tests/system/isctest/check.py
index 054f0f8b76..21c3988dc5 100644
--- a/bin/tests/system/isctest/check.py
+++ b/bin/tests/system/isctest/check.py
@@ -34,6 +34,10 @@ def noerror(message: dns.message.Message) -> None:
     rcode(message, dns_rcode.NOERROR)
 
 
+def servfail(message: dns.message.Message) -> None:
+    rcode(message, dns_rcode.SERVFAIL)
+
+
 def rrsets_equal(first_rrset: dns.rrset.RRset, second_rrset: dns.rrset.RRset) -> None:
     for rr in first_rrset:
         assert rr in second_rrset