A related PROBLEM NOTE on SAS.COM support website (I used SEARCH to find it). You are using a numeric SAS variable "N" against a character SAS format, which is causing the WARNING message condition.
Suggest you make a code tightening correction (my words) and use a PUT function against the "N" NUMERIC type variable with consideration for the length of &NUMGRPS, as shown below:
PUT(N,5. -L )
Problem Note 35428: Warning messages that incorrectly produce a RC=0 now correctly produce a RC=4 in SAS 9.2
http://support.sas.com/kb/35/428.html