diff --git a/.changeset/light-pumas-heal.md b/.changeset/light-pumas-heal.md
new file mode 100644
index 000000000000..d518a3dbca04
--- /dev/null
+++ b/.changeset/light-pumas-heal.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Handle non-compliant ReadableStream implementations
diff --git a/packages/kit/src/runtime/server/endpoint.js b/packages/kit/src/runtime/server/endpoint.js
index 794217e2cb55..94ba79f20d23 100644
--- a/packages/kit/src/runtime/server/endpoint.js
+++ b/packages/kit/src/runtime/server/endpoint.js
@@ -116,7 +116,7 @@ function is_pojo(body) {
 		if (body._readableState && body._writableState && body._events) return false;
 
 		// similarly, it could be a web ReadableStream
-		if (body[Symbol.toStringTag] === 'ReadableStream') return false;
+		if (typeof ReadableStream !== 'undefined' && body instanceof ReadableStream) return false;
 	}
 
 	return true;