diff --git a/test/parallel/test-debugger-client-addhandle.js b/test/parallel/test-debugger-client-addhandle.js
new file mode 100644
index 00000000000000..bb953561b945a9
--- /dev/null
+++ b/test/parallel/test-debugger-client-addhandle.js
@@ -0,0 +1,44 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const Client = require('_debugger').Client;
+
+{
+  const client = new Client();
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle(null);
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle('not an object');
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle({ handle: 'not a number' });
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  const validNoScript = { handle: 6, id: 'foo', type: 'not a script' };
+  client._addHandle(validNoScript);
+  assert.deepStrictEqual(client.handles, { 6: validNoScript });
+  assert.deepStrictEqual(client.scripts, {});
+}
+
+{
+  const client = new Client();
+  const validWithScript = { handle: 5, id: 'bar', type: 'script' };
+  client._addHandle(validWithScript);
+  assert.deepStrictEqual(client.handles, { 5: validWithScript });
+  assert.deepStrictEqual(client.scripts, { bar: validWithScript });
+}