diff --git a/src/anki.ts b/src/anki.ts
index 10a79044..9bbdda78 100644
--- a/src/anki.ts
+++ b/src/anki.ts
@@ -34,6 +34,7 @@ export function invoke(action: string, params={}) {
         });
 
         xhr.open('POST', 'http://127.0.0.1:' + ANKI_PORT.toString());
+        xhr.setRequestHeader('Content-Type', 'application/json');
         xhr.send(JSON.stringify({action, version: 6, params}));
     });
 }