diff --git a/sync3/handler/handler.go b/sync3/handler/handler.go
index 9ab0db77..d3b5bb89 100644
--- a/sync3/handler/handler.go
+++ b/sync3/handler/handler.go
@@ -185,7 +185,12 @@ func (h *SyncLiveHandler) addPrometheusMetrics() {
 
 func (h *SyncLiveHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	if req.Method != "POST" {
+		defer req.Body.Close()
+		w.Header().Add("Content-Type", "application/json")
 		w.WriteHeader(http.StatusMethodNotAllowed)
+		// This is a standard error response.
+		// See M_UNRECOGNIZED in https://spec.matrix.org/v1.10/client-server-api/#common-error-codes
+		w.Write([]byte("{\"errcode\":\"M_UNRECOGNIZED\",\"error\":\"Wrong request method\"}"))
 		return
 	}
 	err := h.serve(w, req)