diff --git a/sippy/ua.go b/sippy/ua.go
index 0712880..501fbe1 100644
--- a/sippy/ua.go
+++ b/sippy/ua.go
@@ -40,12 +40,17 @@ import (
     "github.com/sippy/go-b2bua/sippy/utils"
 )
 
+type SessionLocker interface {
+    sync.Locker
+    TryLock() bool
+}
+
 type Ua struct {
     sip_tm          sippy_types.SipTransactionManager
     sip_tm_lock     sync.RWMutex
     config          sippy_conf.Config
     call_controller sippy_types.CallController
-    session_lock    sync.Locker
+    session_lock    SessionLocker
     state           sippy_types.UaState
     equeue          []sippy_types.CCEvent
     elast_seq       int64
@@ -131,7 +136,7 @@ func (self *Ua) String() string {
     return "UA state: " + self.state.String() + ", Call-Id: " + self.cId.CallId
 }
 
-func NewUA(sip_tm sippy_types.SipTransactionManager, config sippy_conf.Config, nh_address *sippy_net.HostPort, call_controller sippy_types.CallController, session_lock sync.Locker, heir sippy_types.UA) *Ua {
+func NewUA(sip_tm sippy_types.SipTransactionManager, config sippy_conf.Config, nh_address *sippy_net.HostPort, call_controller sippy_types.CallController, session_lock SessionLocker, heir sippy_types.UA) *Ua {
     return &Ua{
         sip_tm          : sip_tm,
         call_controller : call_controller,
@@ -222,9 +227,17 @@ 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) {
+            if self.session_lock.TryLock() {
+                self.session_lock.Unlock()
+                panic("self.session_lock is not locked")
+            }
+            cancel_cb(rtime, req)
+        }
         return &sippy_types.Ua_context{
             Response : nil,
-            CancelCB : self.state.RecvCancel,
+            CancelCB : cancel_fn,
             NoAckCB  : disc_fn,
         }
     } else {