Fix the bug of pressing cancel button in invitation dialog
diff --git a/src/contactpanel.cpp b/src/contactpanel.cpp
index 57d8c80..4d281d6 100644
--- a/src/contactpanel.cpp
+++ b/src/contactpanel.cpp
@@ -623,7 +623,7 @@
 void
 ContactPanel::rejectInvitation(const ChronosInvitation& invitation)
 {
-  string empty;
+  string empty("nack");
   m_handler->publishDataByIdentity (invitation.getInterestName(), empty);
 }