diff --git a/CHANGES b/CHANGES
index d1522b072f..8b4d642693 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,10 @@
+4187.	[func]		When any RR type implementation doesn't
+			implement totext() for the RDATA's wire
+			representation and returns ISC_R_NOTIMPLEMENTED,
+			such RDATA is now printed in unknown
+			presentation format (RFC 3597). RR types affected
+			include LOC(29) and APL(42). [RT #40317].
+
 4186.	[bug]		Fixed an RPZ bug where a QNAME would be matched
 			against a policy RR with wildcard owner name
 			(trigger) where the QNAME was the wildcard owner
diff --git a/lib/dns/rdata.c b/lib/dns/rdata.c
index 24c8bdf323..f6871aae21 100644
--- a/lib/dns/rdata.c
+++ b/lib/dns/rdata.c
@@ -837,6 +837,7 @@ rdata_totext(dns_rdata_t *rdata, dns_rdata_textctx_t *tctx,
 {
 	isc_result_t result = ISC_R_NOTIMPLEMENTED;
 	isc_boolean_t use_default = ISC_FALSE;
+	unsigned int cur;
 
 	REQUIRE(rdata != NULL);
 	REQUIRE(tctx->origin == NULL ||
@@ -850,10 +851,17 @@ rdata_totext(dns_rdata_t *rdata, dns_rdata_textctx_t *tctx,
 		return (ISC_R_SUCCESS);
 	}
 
+	cur = isc_buffer_usedlength(target);
+
 	TOTEXTSWITCH
 
-	if (use_default)
+	if (use_default || (result == ISC_R_NOTIMPLEMENTED)) {
+		unsigned int u = isc_buffer_usedlength(target);
+
+		INSIST(u >= cur);
+		isc_buffer_subtract(target, u - cur);
 		result = unknown_totext(rdata, tctx, target);
+	}
 
 	return (result);
 }
diff --git a/lib/dns/rdata/generic/loc_29.c b/lib/dns/rdata/generic/loc_29.c
index 89b53a309b..641daca25d 100644
--- a/lib/dns/rdata/generic/loc_29.c
+++ b/lib/dns/rdata/generic/loc_29.c
@@ -480,7 +480,11 @@ totext_loc(ARGS_TOTEXT) {
 
 	dns_rdata_toregion(rdata, &sr);
 
-	/* version = sr.base[0]; */
+	if (sr.base[0] != 0)
+		return (ISC_R_NOTIMPLEMENTED);
+
+	REQUIRE(rdata->length == 16);
+
 	size = sr.base[1];
 	INSIST((size&0x0f) < 10 && (size>>4) < 10);
 	if ((size&0x0f)> 1)
@@ -573,8 +577,11 @@ fromwire_loc(ARGS_FROMWIRE) {
 	isc_buffer_activeregion(source, &sr);
 	if (sr.length < 1)
 		return (ISC_R_UNEXPECTEDEND);
-	if (sr.base[0] != 0)
-		return (ISC_R_NOTIMPLEMENTED);
+	if (sr.base[0] != 0) {
+		/* Treat as unknown. */
+		isc_buffer_forward(source, sr.length);
+		return (mem_tobuffer(target, sr.base, sr.length));
+	}
 	if (sr.length < 16)
 		return (ISC_R_UNEXPECTEDEND);