diff --git a/sippy/ua.go b/sippy/ua.go
index 0712880..16fc587 100644
--- a/sippy/ua.go
+++ b/sippy/ua.go
@@ -222,9 +222,15 @@ REQ_LOOP:
         if self.pr_rel {
             t.SetPrackCBs(self.RecvPRACK, disc_fn)
         }
+        cancel_cb := self.state.RecvCancel
+        cancel_fn := func(rtime *sippy_time.MonoTime, req sippy_types.SipRequest) {
+            self.session_lock.Lock()
+            cancel_cb(rtime, req)
+            self.session_lock.Unlock()
+        }
         return &sippy_types.Ua_context{
             Response : nil,
-            CancelCB : self.state.RecvCancel,
+            CancelCB : cancel_fn,
             NoAckCB  : disc_fn,
         }
     } else {