diff --git a/ext/dom/dom_iterators.c b/ext/dom/dom_iterators.c
index 8c6b3023cca..e292728de27 100644
--- a/ext/dom/dom_iterators.c
+++ b/ext/dom/dom_iterators.c
@@ -132,7 +132,7 @@ static void php_dom_iterator_current_key(zend_object_iterator *iter, zval *key)
 	/* Only dtd named node maps, i.e. the ones based on a libxml hash table or attribute collections,
 	 * are keyed by the name because in that case the name is unique. */
 	if (!objmap->ht && objmap->nodetype != XML_ATTRIBUTE_NODE) {
-		ZVAL_LONG(key, iter->index);
+		ZVAL_LONG(key, iterator->index);
 	} else {
 		dom_object *intern = Z_DOMOBJ_P(&iterator->curobj);
 
@@ -179,6 +179,8 @@ static void php_dom_iterator_move_forward(zend_object_iterator *iter) /* {{{ */
 		return;
 	}
 
+	iterator->index++;
+
 	dom_object *intern = Z_DOMOBJ_P(&iterator->curobj);
 	dom_nnodemap_object *objmap = php_dom_iterator_get_nnmap(iterator);
 
@@ -203,7 +205,7 @@ static void php_dom_iterator_move_forward(zend_object_iterator *iter) /* {{{ */
 						php_dom_mark_cache_tag_up_to_date_from_doc_ref(&iterator->cache_tag, intern->document);
 						curnode = dom_fetch_first_iteration_item(objmap);
 						zend_ulong index = 0;
-						while (curnode != NULL && index++ < iter->index) {
+						while (curnode != NULL && index++ < iterator->index) {
 							curnode = curnode->next;
 						}
 					} else {
@@ -224,15 +226,15 @@ static void php_dom_iterator_move_forward(zend_object_iterator *iter) /* {{{ */
 						previndex = 0;
 						curnode = php_dom_first_child_of_container_node(basenode);
 					} else {
-						previndex = iter->index - 1;
+						previndex = iterator->index - 1;
 						curnode = (xmlNodePtr)((php_libxml_node_ptr *)intern->ptr)->node;
 					}
 					curnode = dom_get_elements_by_tag_name_ns_raw(
-						basenode, curnode, objmap->ns, objmap->local, objmap->local_lower, &previndex, iter->index);
+						basenode, curnode, objmap->ns, objmap->local, objmap->local_lower, &previndex, iterator->index);
 				}
 			}
 		} else {
-			curnode = php_dom_libxml_hash_iter(objmap, iter->index);
+			curnode = php_dom_libxml_hash_iter(objmap, iterator->index);
 		}
 	}
 
diff --git a/ext/dom/php_dom.h b/ext/dom/php_dom.h
index 1c21d8a64e1..438857305db 100644
--- a/ext/dom/php_dom.h
+++ b/ext/dom/php_dom.h
@@ -98,6 +98,9 @@ typedef struct {
 	zend_object_iterator intern;
 	zval curobj;
 	HashPosition pos;
+	/* intern->index is only updated for FE_* opcodes, not for e.g. unpacking,
+	 * yet we need to track the position of the node relative to the start. */
+	zend_ulong index;
 	php_libxml_cache_tag cache_tag;
 } php_dom_iterator;
 
diff --git a/ext/dom/tests/modern/common/unpacking_foreach.phpt b/ext/dom/tests/modern/common/unpacking_foreach.phpt
new file mode 100644
index 00000000000..b2bb301ccdc
--- /dev/null
+++ b/ext/dom/tests/modern/common/unpacking_foreach.phpt
@@ -0,0 +1,30 @@
+--TEST--
+unpacking vs foreach in new DOM
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$html = Dom\HTMLDocument::createFromString
+(
+    source: "<h1>Hi</h1><p>hi</p>",
+    options: LIBXML_NOERROR,
+);
+
+foreach ($html->body->childNodes as $node) {
+    echo $node->localName, "\n";
+}
+
+echo "---\n";
+
+foreach ([...$html->body->childNodes] as $node) {
+    echo $node->localName, "\n";
+}
+
+?>
+--EXPECT--
+h1
+p
+---
+h1
+p
diff --git a/ext/dom/tests/unpack_foreach_behaviour.phpt b/ext/dom/tests/unpack_foreach_behaviour.phpt
new file mode 100644
index 00000000000..42fe896d9f7
--- /dev/null
+++ b/ext/dom/tests/unpack_foreach_behaviour.phpt
@@ -0,0 +1,31 @@
+--TEST--
+Unpacking vs foreach behaviour
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$dom = new DOMDocument;
+$dom->loadXML('<root><a/><b/></root>');
+
+echo "--- By foreach: ---\n";
+
+foreach ($dom->documentElement->getElementsByTagName('*') as $node) {
+    var_dump($node->localName);
+}
+
+echo "--- By unpacking: ---\n";
+
+$iter = $dom->documentElement->getElementsByTagName('*');
+foreach ([...$iter] as $node) {
+    var_dump($node->localName);
+}
+
+?>
+--EXPECT--
+--- By foreach: ---
+string(1) "a"
+string(1) "b"
+--- By unpacking: ---
+string(1) "a"
+string(1) "b"
diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index d38e2193d63..7f0cbaeb346 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -1573,7 +1573,9 @@ PHP_FUNCTION(mb_output_handler)
 		char *mimetype = NULL;
 
 		/* Analyze mime type */
-		if (SG(sapi_headers).mimetype && _php_mb_match_regex(MBSTRG(http_output_conv_mimetypes), SG(sapi_headers).mimetype, strlen(SG(sapi_headers).mimetype))) {
+		if (SG(sapi_headers).mimetype
+		 && MBSTRG(http_output_conv_mimetypes)
+		 && _php_mb_match_regex(MBSTRG(http_output_conv_mimetypes), SG(sapi_headers).mimetype, strlen(SG(sapi_headers).mimetype))) {
 			char *s;
 			if ((s = strchr(SG(sapi_headers).mimetype, ';')) == NULL) {
 				mimetype = estrdup(SG(sapi_headers).mimetype);
diff --git a/ext/mbstring/tests/gh17989.phpt b/ext/mbstring/tests/gh17989.phpt
new file mode 100644
index 00000000000..40efd5866c1
--- /dev/null
+++ b/ext/mbstring/tests/gh17989.phpt
@@ -0,0 +1,16 @@
+--TEST--
+GH-17989 (mb_output_handler crash with unset http_output_conv_mimetypes)
+--EXTENSIONS--
+mbstring
+--INI--
+mbstring.http_output_conv_mimetypes=
+--FILE--
+<?php
+echo "set mime type via this echo\n";
+ob_start('mb_output_handler');
+echo "hi";
+ob_flush();
+?>
+--EXPECT--
+set mime type via this echo
+hi