Problems with values in INSERT-Command

If I do the command

INSERT INTO TBL (f1) VALUES (2); 
instead of 
INSERT INTO TBL (f1) VALUES ('2');

I get an error-message "fmgr_info: function text: not in internal table". 

According to the documentation, it must be possible, to put numeric values
without quotes, so what is the reason for the error. It happens in all other
cases, but sometimes even with worse error-messages. Any ideas?

Michael