Index: contrib/bind9/lib/dns/rdata.c
===================================================================
--- contrib/bind9/lib/dns/rdata.c.orig
+++ contrib/bind9/lib/dns/rdata.c
@@ -329,8 +329,8 @@
 
 	REQUIRE(rdata1 != NULL);
 	REQUIRE(rdata2 != NULL);
-	REQUIRE(rdata1->data != NULL);
-	REQUIRE(rdata2->data != NULL);
+	REQUIRE(rdata1->length == 0 || rdata1->data != NULL);
+	REQUIRE(rdata2->length == 0 || rdata2->data != NULL);
 	REQUIRE(DNS_RDATA_VALIDFLAGS(rdata1));
 	REQUIRE(DNS_RDATA_VALIDFLAGS(rdata2));
 
@@ -360,8 +360,8 @@
 
 	REQUIRE(rdata1 != NULL);
 	REQUIRE(rdata2 != NULL);
-	REQUIRE(rdata1->data != NULL);
-	REQUIRE(rdata2->data != NULL);
+	REQUIRE(rdata1->length == 0 || rdata1->data != NULL);
+	REQUIRE(rdata2->length == 0 || rdata2->data != NULL);
 	REQUIRE(DNS_RDATA_VALIDFLAGS(rdata1));
 	REQUIRE(DNS_RDATA_VALIDFLAGS(rdata2));
 
Index: contrib/bind9/lib/dns/rdataslab.c
===================================================================
--- contrib/bind9/lib/dns/rdataslab.c.orig
+++ contrib/bind9/lib/dns/rdataslab.c
@@ -126,6 +126,11 @@
 dns_rdataslab_fromrdataset(dns_rdataset_t *rdataset, isc_mem_t *mctx,
 			   isc_region_t *region, unsigned int reservelen)
 {
+	/*
+	 * Use &removed as a sentinal pointer for duplicate
+	 * rdata as rdata.data == NULL is valid.
+	 */
+	static unsigned char removed;
 	struct xrdata  *x;
 	unsigned char  *rawbuf;
 #if DNS_RDATASET_FIXED
@@ -169,6 +174,7 @@
 		INSIST(result == ISC_R_SUCCESS);
 		dns_rdata_init(&x[i].rdata);
 		dns_rdataset_current(rdataset, &x[i].rdata);
+		INSIST(x[i].rdata.data != &removed);
 #if DNS_RDATASET_FIXED
 		x[i].order = i;
 #endif
@@ -201,8 +207,7 @@
 	 */
 	for (i = 1; i < nalloc; i++) {
 		if (compare_rdata(&x[i-1].rdata, &x[i].rdata) == 0) {
-			x[i-1].rdata.data = NULL;
-			x[i-1].rdata.length = 0;
+			x[i-1].rdata.data = &removed;
 #if DNS_RDATASET_FIXED
 			/*
 			 * Preserve the least order so A, B, A -> A, B
@@ -292,7 +297,7 @@
 #endif
 
 	for (i = 0; i < nalloc; i++) {
-		if (x[i].rdata.data == NULL)
+		if (x[i].rdata.data == &removed)
 			continue;
 #if DNS_RDATASET_FIXED
 		offsettable[x[i].order] = rawbuf - offsetbase;