If RHEL exit codes are positive numbers, how can a call system in SQR return -1?

We have an SQR program that uses Call System to run SFTP.  It pulls some files from a remote server.  We have been using this for years on HP-UX, but are now transitioning to RHEL 6.  One that platform we are seeing some odd behavior.

It connects successfully to the remove server, pulls the files, and terminates the FTP program.  Everything is working as it should, except that the Call System returns a -1 status.  We have to check the status because occasionally one of the files will be missing on the remote server, and we need to catch that and terminate the job.