diff --git a/src/sync-manager.ts b/src/sync-manager.ts
index 9ba942f..009b672 100644
--- a/src/sync-manager.ts
+++ b/src/sync-manager.ts
@@ -441,11 +441,16 @@ class PushLoop extends Loop {
 
     let response;
     try {
-      response = await fetch(url, {
+      const options = {
         method,
-        body: value ? JSON.stringify(value) : undefined,
+        headers: {},
         ...this.opts.fetchOptions,
-      });
+      }
+      if (value) {
+        options.body = JSON.stringify(value);
+        options.headers['Content-Type'] = 'application/json';
+      }
+      response = await fetch(url, options);
     } catch (e) {
       return retryAfter(DEFAULT_RETRY_DELAY);
     }
diff --git a/test/sync-server.js b/test/sync-server.js
index b04774c..948a800 100644
--- a/test/sync-server.js
+++ b/test/sync-server.js
@@ -44,6 +44,7 @@ const server = createServer(async (req, res) => {
   switch (req.method) {
     case 'OPTIONS':
       res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PUT,DELETE');
+      res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
       res.writeHead(204).end();
       break;
     case 'GET':