An ISAM file open (such as OPNRFIL, OPNRFILX, OPNIFIL, or OPNIFILX) could fail with a DMAP_ERR (957) error if the timing was right when the ISAM file open/close calls overlapped with OPNRFILX calling CLSFIL on data file. This situation only occurs under specific timing and overlapping of ISAM open and close operations by multiple connections. Logic has been modified to prevent this error.