Liberate for library file

Hello, everyone! I have a problem with the usage of Liberate.  The error information is 

*Warning* (write_verilog) : No function written for pin Q of cell balloon_sim_nognd

Then, I look at the verilog file, some information like that 

module balloon_sim_nognd (Q, D, NRESTORE, SAVE, CK);
output Q;
input D, NRESTORE, SAVE, CK;
reg notifier;
wire delayed_D, delayed_NRESTORE, delayed_SAVE, delayed_CK;
// Missing function for pin Q
// Timing

I don't know the reason.