diff --git a/lib/adapters/fetch.js b/lib/adapters/fetch.js
index 3abd8cf2e2..1104553581 100644
--- a/lib/adapters/fetch.js
+++ b/lib/adapters/fetch.js
@@ -17,11 +17,11 @@ const fetchProgressDecorator = (total, fn) => {
   }));
 }
 
-const isFetchSupported = typeof fetch !== 'undefined';
-const isReadableStreamSupported = isFetchSupported && typeof ReadableStream !== 'undefined';
+const isFetchSupported = typeof fetch === 'function' && typeof Request === 'function' && typeof Response === 'function';
+const isReadableStreamSupported = isFetchSupported && typeof ReadableStream === 'function';
 
 // used only inside the fetch adapter
-const encodeText = isFetchSupported && (typeof TextEncoder !== 'undefined' ?
+const encodeText = isFetchSupported && (typeof TextEncoder === 'function' ?
     ((encoder) => (str) => encoder.encode(str))(new TextEncoder()) :
     async (str) => new Uint8Array(await new Response(str).arrayBuffer())
 );