diff --git a/lib/isc/hashmap.c b/lib/isc/hashmap.c
index a8e635a969..1fa5460770 100644
--- a/lib/isc/hashmap.c
+++ b/lib/isc/hashmap.c
@@ -87,6 +87,7 @@ struct isc_hashmap {
 	isc_mem_t *mctx;
 	size_t count;
 	hashmap_table_t tables[HASHMAP_NUM_TABLES];
+	uint_fast32_t iterators;
 };
 
 struct isc_hashmap_iter {
@@ -350,6 +351,9 @@ hashmap_rehash_one(isc_hashmap_t *hashmap) {
 	hashmap_node_t *oldtable = hashmap->tables[oldidx].table;
 	hashmap_node_t node;
 
+	/* Don't rehash when iterating */
+	INSIST(hashmap->iterators == 0);
+
 	/* Find first non-empty node */
 	while (hashmap->hiter < oldsize && oldtable[hashmap->hiter].key == NULL)
 	{
@@ -502,6 +506,8 @@ hashmap_add(isc_hashmap_t *hashmap, const uint32_t hashval,
 	hashmap_node_t *current = NULL;
 	uint32_t pos;
 
+	INSIST(hashmap->iterators == 0);
+
 	hash = isc_hash_bits32(hashval, hashmap->tables[idx].hashbits);
 
 	/* Initialize the node to be store to 'node' */
@@ -599,6 +605,8 @@ isc_hashmap_iter_create(isc_hashmap_t *hashmap, isc_hashmap_iter_t **iterp) {
 		.hindex = hashmap->hindex,
 	};
 
+	hashmap->iterators++;
+
 	*iterp = iter;
 }
 
@@ -613,6 +621,9 @@ isc_hashmap_iter_destroy(isc_hashmap_iter_t **iterp) {
 	*iterp = NULL;
 	hashmap = iter->hashmap;
 	isc_mem_put(hashmap->mctx, iter, sizeof(*iter));
+
+	INSIST(hashmap->iterators > 0);
+	hashmap->iterators--;
 }
 
 static isc_result_t