diff --git a/lib/api/api-fetch.js b/lib/api/api-fetch.js
index 3720719c155..a0d7107f01f 100644
--- a/lib/api/api-fetch.js
+++ b/lib/api/api-fetch.js
@@ -15,7 +15,6 @@ const util = require('../core/util')
 const { AsyncResource } = require('async_hooks')
 const { addSignal, removeSignal } = require('./abort-signal')
 const { isBlob } = require('buffer')
-const assert = require('assert')
 
 const kStatus = Symbol('status')
 const kUrlList = Symbol('url list')
@@ -300,6 +299,7 @@ function normalizeAndValidateRequestMethod (method) {
   return normalizedMethod
 }
 
+// https://fetch.spec.whatwg.org/#concept-bodyinit-extract
 function extractBody (body) {
   // TODO: FormBody
 
@@ -322,13 +322,11 @@ function extractBody (body) {
     typeof body.pipeTo === 'function' // web ReadableStream
   ) {
     if (body.locked) {
-      // TODO: Standard says "Assert" but is unclear regarding what that means?
-      assert(false, 'locked')
+      throw new TypeError('locked')
     }
 
     if (util.isDisturbed(body)) {
-      // TODO: Standard says "Assert" but is unclear regarding what that means?
-      assert(false, 'disturbed')
+      throw new TypeError('disturbed')
     }
 
     return [body, null]