Fix safe handling of constructor null values

.gitignore

@@ -153,3 +153,9 @@ yarn.lock
 
 **/._*
 **/*.pem
+
+# CLAUDE.md
+CLAUDE.md
+
+# Claude Code directory
+.claude

index.js

@@ -76,6 +76,8 @@ function filter (obj, { protoAction = 'error', constructorAction = 'error', safe
 
       if (constructorAction !== 'ignore' &&
           Object.prototype.hasOwnProperty.call(node, 'constructor') &&
+          node.constructor !== null &&
+          typeof node.constructor === 'object' &&
           Object.prototype.hasOwnProperty.call(node.constructor, 'prototype')) { // Avoid calling node.hasOwnProperty directly
         if (safe === true) {
           return null

test/index.test.js

@@ -257,6 +257,27 @@ test('parse', t => {
       t.end()
     })
 
+    t.test('handles constructor null safely', t => {
+      // Test that constructor: null doesn't trigger prototype pollution checks
+      t.deepEqual(
+        j.parse('{"constructor": null}', { constructorAction: 'remove' }),
+        { constructor: null }
+      )
+
+      // Test that constructor: null doesn't throw error when using error action
+      t.deepEqual(
+        j.parse('{"constructor": null}', { constructorAction: 'error' }),
+        { constructor: null }
+      )
+
+      // Test that constructor: null is preserved when using ignore action
+      t.deepEqual(
+        j.parse('{"constructor": null}', { constructorAction: 'ignore' }),
+        { constructor: null }
+      )
+      t.end()
+    })
+
     t.end()
   })