diff --git a/hemx-js/runtime/hemx.js b/hemx-js/runtime/hemx.js
index 97cdaec..7c31dbd 100644
--- a/hemx-js/runtime/hemx.js
+++ b/hemx-js/runtime/hemx.js
@@ -153,7 +153,7 @@
   function urlEncoded(data) {
     const encoded = new URLSearchParams();
     for (const [name, value] of data.entries()) {
-      if (value instanceof File) continue;
+      if (typeof File !== "undefined" && value instanceof File) continue;
       encoded.append(name, value);
     }
     return encoded;
diff --git a/hemx-js/tests/runtime.rs b/hemx-js/tests/runtime.rs
index df2ee45..47dee5c 100644
--- a/hemx-js/tests/runtime.rs
+++ b/hemx-js/tests/runtime.rs
@@ -17,7 +17,7 @@ fn runtime_preserves_multipart_file_upload_fallback_shape() {
     assert!(source.contains(
         "multipart: form && String(form.enctype).toLowerCase() === \"multipart/form-data\""
     ));
-    assert!(source.contains("if (value instanceof File) continue"));
+    assert!(source.contains("if (typeof File !== \"undefined\" && value instanceof File) continue"));
     assert!(source.contains("return multipart ? data : urlEncoded(data)"));
     assert!(source.contains("if (body instanceof URLSearchParams) headers[\"Content-Type\"] = \"application/x-www-form-urlencoded;charset=UTF-8\""));
 }