diff --git a/basicswap/http_server.py b/basicswap/http_server.py
index a4e38d5..386aa2f 100644
--- a/basicswap/http_server.py
+++ b/basicswap/http_server.py
@@ -649,7 +649,7 @@ class HttpHandler(BaseHTTPRequestHandler):
                     self.wfile.write(
                         json.dumps({"error": "Unauthorized"}).encode("utf-8")
                     )
-                    return
+                    return b""
                 else:
                     self.send_response(302)
                     self.send_header("Location", "/login")