diff --git a/lib/dns/rdata/generic/zonemd_63.c b/lib/dns/rdata/generic/zonemd_63.c
index 8148593a85..604a901d3e 100644
--- a/lib/dns/rdata/generic/zonemd_63.c
+++ b/lib/dns/rdata/generic/zonemd_63.c
@@ -70,7 +70,7 @@ totext_zonemd(ARGS_TOTEXT) {
 	char buf[sizeof("0123456789")];
 	unsigned long num;
 
-	REQUIRE(rdata->length != 0);
+	REQUIRE(rdata->length > 6);
 
 	UNUSED(tctx);
 
@@ -136,9 +136,13 @@ fromwire_zonemd(ARGS_FROMWIRE) {
 	isc_buffer_activeregion(source, &sr);
 
 	/*
-	 * Check digest lengths if we know them.
+	 * If we do not recognize the digest type, only ensure that the digest
+	 * is present at all.
+	 *
+	 * If we do recognize the digest type, ensure that the digest is of the
+	 * correct length.
 	 */
-	if (sr.length < 6 ||
+	if (sr.length < 7 ||
 	    (sr.base[4] == DNS_ZONEMD_DIGEST_SHA384 &&
 	     sr.length < 6 + ISC_SHA384_DIGESTLENGTH))
 	{
@@ -146,9 +150,10 @@ fromwire_zonemd(ARGS_FROMWIRE) {
 	}
 
 	/*
-	 * Only copy digest lengths if we know them.
-	 * If there is extra data dns_rdata_fromwire() will
-	 * detect that.
+	 * Only specify the number of octets to consume if we recognize the
+	 * digest type.
+	 *
+	 * If there is extra data, dns_rdata_fromwire() will detect that.
 	 */
 	if (sr.base[4] == DNS_ZONEMD_DIGEST_SHA384) {
 		sr.length = 6 + ISC_SHA384_DIGESTLENGTH;
diff --git a/lib/dns/tests/rdata_test.c b/lib/dns/tests/rdata_test.c
index 3bb104f0b8..0e8e2763d8 100644
--- a/lib/dns/tests/rdata_test.c
+++ b/lib/dns/tests/rdata_test.c
@@ -1642,7 +1642,7 @@ zonemd(void **state) {
 		TEXT_INVALID("0 0"),
 		TEXT_INVALID("0 0 0"),
 		TEXT_INVALID("99999999 0 0"),
-		TEXT_INVALID("2019020700 0 0 "),
+		TEXT_INVALID("2019020700 0 0"),
 		TEXT_INVALID("2019020700 1 0 DEADBEEF"),
 		TEXT_VALID("2019020700 2 0 DEADBEEF"),
 		TEXT_VALID("2019020700 255 0 DEADBEEF"),
@@ -1676,26 +1676,20 @@ zonemd(void **state) {
 		 */
 		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x00),
 		/*
-		 * Serial + type + reserved only - digest type
-		 * undefined, so we accept the missing digest.
+		 * Short.
 		 */
-		WIRE_VALID(0x00, 0x00, 0x00, 0x00, 0x00, 0x00),
+		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x00, 0x00),
 		/*
-		 * SHA-384 is defined, so we insist there be a digest.
+		 * Minimal, one-octet hash for an undefined digest type.
 		 */
-		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x01, 0x00),
+		WIRE_VALID(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00),
 		/*
-		 * Four octets, too short for SHA-384.
+		 * SHA-384 is defined, so we insist there be a digest of
+		 * the expected length.
 		 */
-		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
-			     0xde, 0xad, 0xbe, 0xef),
+		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00),
 		/*
-		 * Digest type undefined, so accept the short digest.
-		 */
-		WIRE_VALID(0x00, 0x00, 0x00, 0x00, 0xff, 0x00,
-			   0xde, 0xad, 0xbe, 0xef),
-		/*
-		 * 48 octets, valid for SHA-384.
+		 * 48-octet digest, valid for SHA-384.
 		 */
 		WIRE_VALID(0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
 			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
@@ -1706,6 +1700,32 @@ zonemd(void **state) {
 			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
 			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
 			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce),
+		/*
+		 * 56-octet digest, too long for SHA-384.
+		 */
+		WIRE_INVALID(0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			     0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce),
+		/*
+		 * 56-octet digest, valid for an undefined digest type.
+		 */
+		WIRE_VALID(0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce,
+			   0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce),
 		/*
 		 * Sentinel.
 		 */