diff --git a/qr2/challenge.go b/qr2/challenge.go
index ad336c5..52940c9 100644
--- a/qr2/challenge.go
+++ b/qr2/challenge.go
@@ -33,7 +33,12 @@ func sendChallenge(conn net.PacketConn, addr net.UDPAddr, session Session, looku
 
 		challenge = common.RandomString(6) + "00" + hexIP + hexPort
 		mutex.Lock()
-		sessions[lookupAddr].Challenge = challenge
+		if sessionPtr := sessions[lookupAddr]; sessionPtr != nil {
+			sessionPtr.Challenge = challenge
+		} else {
+			mutex.Unlock()
+			return
+		}
 		mutex.Unlock()
 	}