OPT_STRING('s', "sort", &sort_order, "key[,key2...]",
                   "sort by key(s): pid, comm, dso, symbol, parent, cpu, srcline, ..."
                   " Please refer the man page for the complete list."),
-       OPT_STRING('t', "field-separator", &symbol_conf.field_sep, "separator",
+       OPT_STRING_NOEMPTY('t', "field-separator", &symbol_conf.field_sep, "separator",
                   "separator for columns, no spaces will be added between "
                   "columns '.' is reserved."),
        OPT_STRING(0, "symfs", &symbol_conf.symfs, "directory",
 
                   "input file name"),
        OPT_STRING('C', "cpu", &mem.cpu_list, "cpu",
                   "list of cpus to profile"),
-       OPT_STRING('x', "field-separator", &symbol_conf.field_sep,
+       OPT_STRING_NOEMPTY('x', "field-separator", &symbol_conf.field_sep,
                   "separator",
                   "separator for columns, no spaces will be added"
                   " between columns '.' is reserved."),