diff --git a/js/modules/loki_rpc.js b/js/modules/loki_rpc.js
index 19e54b71a..a61422506 100644
--- a/js/modules/loki_rpc.js
+++ b/js/modules/loki_rpc.js
@@ -106,7 +106,10 @@ const rpc = (address, port, method, params, options = {}) => {
     method: 'POST',
     ...options,
     body: JSON.stringify(body),
-    headers,
+    headers: {
+      'Content-Type': 'application/json',
+      ...headers,
+    },
   };
 
   return fetch(url, fetchOptions);