diff --git a/ext/standard/tests/serialize/incomplete_class_c_format.phpt b/ext/standard/tests/serialize/incomplete_class_c_format.phpt
new file mode 100644
index 000000000000..258e76921c4a
--- /dev/null
+++ b/ext/standard/tests/serialize/incomplete_class_c_format.phpt
@@ -0,0 +1,36 @@
+--TEST--
+unserialize() turns C: objects of missing classes into incomplete objects
+--FILE--
+<?php
+
+var_dump(unserialize('a:2:{i:0;C:7:"Missing":4:{abcd}i:1;s:4:"tail";}'));
+
+function callback(string $class): void
+{
+    echo __FUNCTION__, "($class)\n";
+}
+
+ini_set('unserialize_callback_func', 'callback');
+
+var_dump(unserialize('C:7:"Missing":4:{abcd}'));
+?>
+--EXPECTF--
+Warning: Class __PHP_Incomplete_Class has no unserializer in %s on line %d
+array(2) {
+  [0]=>
+  object(__PHP_Incomplete_Class)#%d (1) {
+    ["__PHP_Incomplete_Class_Name"]=>
+    string(7) "Missing"
+  }
+  [1]=>
+  string(4) "tail"
+}
+callback(Missing)
+
+Warning: unserialize(): Function callback() hasn't defined the class it was called for in %s on line %d
+
+Warning: Class __PHP_Incomplete_Class has no unserializer in %s on line %d
+object(__PHP_Incomplete_Class)#%d (1) {
+  ["__PHP_Incomplete_Class_Name"]=>
+  string(7) "Missing"
+}
diff --git a/ext/standard/tests/serialize/incomplete_class_c_format_allowed_classes.phpt b/ext/standard/tests/serialize/incomplete_class_c_format_allowed_classes.phpt
new file mode 100644
index 000000000000..54a68a935ef1
--- /dev/null
+++ b/ext/standard/tests/serialize/incomplete_class_c_format_allowed_classes.phpt
@@ -0,0 +1,63 @@
+--TEST--
+unserialize() with allowed_classes turns C: objects of classes that are not allowed into incomplete objects
+--FILE--
+<?php
+
+final class Kept
+{
+    public $a = 1;
+}
+
+final class Blocked implements Serializable
+{
+    public function serialize(): string
+    {
+        return '';
+    }
+
+    public function unserialize(string $data): void
+    {
+        echo __METHOD__, "\n";
+    }
+
+    public function __serialize(): array
+    {
+        return [];
+    }
+
+    public function __unserialize(array $data): void
+    {
+        echo __METHOD__, "\n";
+    }
+}
+
+$payload = 'a:4:{i:0;O:4:"Kept":1:{s:1:"a";i:2;}i:1;C:7:"Blocked":4:{abcd}i:2;C:7:"Missing":4:{abcd}i:3;r:4;}';
+
+var_dump(unserialize($payload, ['allowed_classes' => ['Kept']]));
+?>
+--EXPECTF--
+Warning: Class __PHP_Incomplete_Class has no unserializer in %s on line %d
+
+Warning: Class __PHP_Incomplete_Class has no unserializer in %s on line %d
+array(4) {
+  [0]=>
+  object(Kept)#%d (1) {
+    ["a"]=>
+    int(2)
+  }
+  [1]=>
+  object(__PHP_Incomplete_Class)#%d (1) {
+    ["__PHP_Incomplete_Class_Name"]=>
+    string(7) "Blocked"
+  }
+  [2]=>
+  object(__PHP_Incomplete_Class)#%d (1) {
+    ["__PHP_Incomplete_Class_Name"]=>
+    string(7) "Missing"
+  }
+  [3]=>
+  object(__PHP_Incomplete_Class)#%d (1) {
+    ["__PHP_Incomplete_Class_Name"]=>
+    string(7) "Blocked"
+  }
+}
diff --git a/ext/standard/tests/serialize/serialization_objects_009.phpt b/ext/standard/tests/serialize/serialization_objects_009.phpt
index 95b85ccd80f7..27da8e53ebb4 100644
--- a/ext/standard/tests/serialize/serialization_objects_009.phpt
+++ b/ext/standard/tests/serialize/serialization_objects_009.phpt
@@ -13,11 +13,12 @@ echo "Done";
 --EXPECTF--
 Warning: Class __PHP_Incomplete_Class has no unserializer in %s on line %d
 
-Warning: unserialize(): Error at offset 11 of 18 bytes in %s on line %d
-
 Warning: Class C has no unserializer in %s on line %d
 
 Warning: unserialize(): Error at offset 11 of 18 bytes in %s on line %d
-bool(false)
+object(__PHP_Incomplete_Class)#%d (1) {
+  ["__PHP_Incomplete_Class_Name"]=>
+  string(1) "C"
+}
 bool(false)
 Done
diff --git a/ext/standard/var_unserializer.re b/ext/standard/var_unserializer.re
index 0138471aa63e..e10b067c68d8 100644
--- a/ext/standard/var_unserializer.re
+++ b/ext/standard/var_unserializer.re
@@ -769,7 +769,11 @@ static inline int object_custom(UNSERIALIZE_PARAMETER, zend_class_entry *ce)
 
 	if (ce->unserialize == NULL) {
 		zend_error(E_WARNING, "Class %s has no unserializer", ZSTR_VAL(ce->name));
-		return 0;
+		/* __PHP_Incomplete_Class has no internal state, an empty instance is safe. */
+		if (ce != PHP_IC_ENTRY) {
+			return 0;
+		}
+		object_init_ex(rval, ce);
 	} else if (ce->unserialize(rval, ce, (const unsigned char*)*p, datalen, (zend_unserialize_data *)var_hash) != SUCCESS) {
 		return 0;
 	}