diff --git a/sorc/chgres_cube.fd/program_setup.f90 b/sorc/chgres_cube.fd/program_setup.f90
index 8e2e3e098..6d1fb37a0 100644
--- a/sorc/chgres_cube.fd/program_setup.f90
+++ b/sorc/chgres_cube.fd/program_setup.f90
@@ -254,7 +254,7 @@ subroutine read_setup_namelist
 !-------------------------------------------------------------------------
 
  is = index(mosaic_file_target_grid, "/", .true.)
- ie = index(mosaic_file_target_grid, "_mosaic")
+ ie = index(mosaic_file_target_grid, "mosaic") - 1
 
  if (is == 0 .or. ie == 0) then
    call error_handler("CANT DETERMINE CRES FROM MOSAIC FILE.", 1)