diff --git a/mm1_modem_proxy.cc b/mm1_modem_proxy.cc
index 017781d..3e4563f 100644
--- a/mm1_modem_proxy.cc
+++ b/mm1_modem_proxy.cc
@@ -18,7 +18,7 @@
 template<typename TraceMsgT, typename CallT, typename CallbackT,
          typename... ArgTypes>
 void ModemProxy::BeginCall(
-    const TraceMsgT &trace_msg, const CallT &call, CallbackT &callback,
+    const TraceMsgT &trace_msg, const CallT &call, const CallbackT &callback,
     Error *error, int timeout, ArgTypes... rest) {
   BeginAsyncDBusCall(trace_msg, proxy_, call, callback, error,
                      &CellularError::FromMM1DBusError, timeout, rest...);