diff --git a/Zend/tests/gh20177.phpt b/Zend/tests/gh20177.phpt
new file mode 100644
index 00000000000..fd69460067f
--- /dev/null
+++ b/Zend/tests/gh20177.phpt
@@ -0,0 +1,25 @@
+--TEST--
+GH-20177: Access overridden private property in get_object_vars()
+--FILE--
+<?php
+
+class A {
+    private $prop = 'A::$prop';
+
+    public function __construct() {
+        var_dump(get_object_vars($this));
+    }
+}
+
+class B extends A {
+    protected $prop = 'B::$prop';
+}
+
+new B;
+
+?>
+--EXPECT--
+array(1) {
+  ["prop"]=>
+  string(8) "A::$prop"
+}
diff --git a/Zend/zend_object_handlers.c b/Zend/zend_object_handlers.c
index 5d1ec65ccda..17676e8edea 100644
--- a/Zend/zend_object_handlers.c
+++ b/Zend/zend_object_handlers.c
@@ -566,6 +566,11 @@ ZEND_API zend_result zend_check_property_access(const zend_object *zobj, zend_st
 				return FAILURE;
 			}
 		} else {
+			/* We were looking for a protected property but found a private one
+			 * belonging to the parent class. */
+			if (property_info->flags & ZEND_ACC_PRIVATE) {
+				return FAILURE;
+			}
 			ZEND_ASSERT(property_info->flags & ZEND_ACC_PROTECTED);
 		}
 		return SUCCESS;