diff --git a/multipart/formfile.ts b/multipart/formfile.ts
index 592f4e529b8f..a0e721a15fc4 100644
--- a/multipart/formfile.ts
+++ b/multipart/formfile.ts
@@ -1,4 +1,5 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+import { hasOwnProperty } from "../util/has_own_property.ts";
 
 /** FormFile object */
 export interface FormFile {
@@ -19,9 +20,5 @@ export interface FormFile {
 /** Type guard for FormFile */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function isFormFile(x: any): x is FormFile {
-  return (
-    typeof x === "object" &&
-    x.hasOwnProperty("filename") &&
-    x.hasOwnProperty("type")
-  );
+  return hasOwnProperty(x, "filename") && hasOwnProperty(x, "type");
 }
diff --git a/multipart/formfile_test.ts b/multipart/formfile_test.ts
index 63509fc9d2b5..52dd3addda69 100644
--- a/multipart/formfile_test.ts
+++ b/multipart/formfile_test.ts
@@ -18,3 +18,15 @@ test(function multipartIsFormFile(): void {
     false
   );
 });
+
+test(function isFormFileShouldNotThrow(): void {
+  assertEquals(
+    isFormFile({
+      filename: "foo",
+      type: "application/json",
+      hasOwnProperty: "bar"
+    }),
+    true
+  );
+  assertEquals(isFormFile(Object.create(null)), false);
+});
diff --git a/util/has_own_property.ts b/util/has_own_property.ts
new file mode 100644
index 000000000000..707d951d3b33
--- /dev/null
+++ b/util/has_own_property.ts
@@ -0,0 +1,30 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * Determines whether an object has a property with the specified name.
+ * Avoid calling prototype builtin `hasOwnProperty` for two reasons:
+ *
+ * 1. `hasOwnProperty` is defined on the object as something else:
+ *
+ *      const options = {
+ *        ending: 'utf8',
+ *        hasOwnProperty: 'foo'
+ *      };
+ *      options.hasOwnProperty('ending') // throws a TypeError
+ *
+ * 2. The object doesn't inherit from `Object.prototype`:
+ *
+ *       const options = Object.create(null);
+ *       options.ending = 'utf8';
+ *       options.hasOwnProperty('ending'); // throws a TypeError
+ *
+ * @param obj A Object.
+ * @param v A property name.
+ * @see https://eslint.org/docs/rules/no-prototype-builtins
+ */
+export function hasOwnProperty<T>(obj: T, v: PropertyKey): boolean {
+  if (obj == null) {
+    return false;
+  }
+  return Object.prototype.hasOwnProperty.call(obj, v);
+}
diff --git a/ws/mod.ts b/ws/mod.ts
index df8cab01f8ec..b649ef1781a2 100644
--- a/ws/mod.ts
+++ b/ws/mod.ts
@@ -1,6 +1,7 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 
 import { decode, encode } from "../strings/mod.ts";
+import { hasOwnProperty } from "../util/has_own_property.ts";
 
 type Conn = Deno.Conn;
 type Writer = Deno.Writer;
@@ -34,7 +35,7 @@ export interface WebSocketCloseEvent {
 export function isWebSocketCloseEvent(
   a: WebSocketEvent
 ): a is WebSocketCloseEvent {
-  return typeof a === "object" && a.hasOwnProperty("code");
+  return hasOwnProperty(a, "code");
 }
 
 export type WebSocketPingEvent = ["ping", Uint8Array];