diff --git a/http/roundtrip_js.go b/http/roundtrip_js.go
index 344b8a3..e693538 100644
--- a/http/roundtrip_js.go
+++ b/http/roundtrip_js.go
@@ -68,9 +68,11 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	// to fall back on the Fetch API, unless it's not available.
 
 	// TINYGO: Dial/DialTLS & DialContext/DialTLSContext are not present in tinygo Transport struct, therefore the
-	// corresponding if statements were removed
+	// corresponding if statements were removed.
+	// TINYGO: t.roundTrip (the private fallback used by upstream Go) is not present in the TinyGo stub
+	// Transport, so return an error when the Fetch API is unavailable instead of calling it.
 	if jsFetchMissing || jsFetchDisabled {
-		return t.roundTrip(req)
+		return nil, errors.New("net/http: Fetch API is not available and no fallback transport is implemented for js/wasm")
 	}
 
 	ac := js.Global().Get("AbortController")