diff --git a/scapy/layers/dns.py b/scapy/layers/dns.py
index 432b0a06066..fe534abbc94 100644
--- a/scapy/layers/dns.py
+++ b/scapy/layers/dns.py
@@ -1138,11 +1138,11 @@ class DNSRRTSIG(_DNSRRdummy):
                    DNSStrField("algo_name", "hmac-sha1"),
                    TimeSignedField("time_signed", 0),
                    ShortField("fudge", 0),
-                   FieldLenField("mac_len", 20, fmt="!H", length_of="mac_data"),  # noqa: E501
+                   FieldLenField("mac_len", None, fmt="!H", length_of="mac_data"),  # noqa: E501
                    StrLenField("mac_data", "", length_from=lambda pkt: pkt.mac_len),  # noqa: E501
                    ShortField("original_id", 0),
                    ShortField("error", 0),
-                   FieldLenField("other_len", 0, fmt="!H", length_of="other_data"),  # noqa: E501
+                   FieldLenField("other_len", None, fmt="!H", length_of="other_data"),  # noqa: E501
                    StrLenField("other_data", "", length_from=lambda pkt: pkt.other_len)  # noqa: E501
                    ]
 
diff --git a/test/scapy/layers/dns_dnssec.uts b/test/scapy/layers/dns_dnssec.uts
index 631f7237921..2882aa3a1f6 100644
--- a/test/scapy/layers/dns_dnssec.uts
+++ b/test/scapy/layers/dns_dnssec.uts
@@ -132,11 +132,20 @@ t.type == 16 and t.rdlen == 312 and t.rdata[0][:5] == b"Scapy" and t.rdata[1][-1
 
 = DNSRRTSIG basic instantiation
 t = DNSRRTSIG()
-raw(t) == b"\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x14\x00\x00\x00\x00\x00\x00"
+raw(t) == b"\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
 
 = DNSRRTSIG(), check parameters
 t = DNSRRTSIG(rrname="SAMPLE-ALG.EXAMPLE.", time_signed=853804800, fudge=300)
-raw(t) == b"\nSAMPLE-ALG\x07EXAMPLE\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x002\xe4\x07\x00\x01,\x00\x14\x00\x00\x00\x00\x00\x00"
+raw(t) == b"\nSAMPLE-ALG\x07EXAMPLE\x00\x00\xfa\x00\x01\x00\x00\x00\x00\x00\x1b\thmac-sha1\x00\x00\x002\xe4\x07\x00\x01,\x00\x00\x00\x00\x00\x00\x00\x00"
+
+= DNSRRTSIG mac_len/other_len auto-computed, round-trips
+# Regression: mac_len/other_len defaulted to a hardcoded 20/0 instead of None,
+# so FieldLenField never auto-computed and a non-empty MAC did not round-trip
+# (the length prefix stayed 20, truncating the MAC to 20 bytes on re-dissect).
+t = DNSRRTSIG(mac_data=b"M" * 32, other_data=b"O" * 6)
+raw_t = raw(t)
+p = DNSRRTSIG(raw_t)
+p.mac_len == 32 and p.other_len == 6 and p.mac_data == b"M" * 32 and p.other_data == b"O" * 6 and raw(p) == raw_t
 
 = TimeField methods