"Bad output 'covered_bases': For input string: "2672692578""

Hi,

Besed on the log files, I think my pipeline is succeeded. But, there is an issue on one of the outputs. I want to attach both the output file and the first line value (Int) in that file to entity attributes. However, there are something wrong with the the first line value. Could you please help me? Any suggestions are welcome. Thanks a lot!

workflow:
https://portal.firecloud.org/#workspaces/broad-4900028-bn10-adam/BN10_test_cbao/monitor/e89ae8eb-a842-44e3-aa19-139c96ad0463/0774c628-41b6-40d9-b157-2ac577a945d1

My WDL:

workflow x_workflow {
    # inputs
    File wc
    String pair_id

    call x {
        input:
            cw = cw,
            pair_id = pair_id,
    }

    output {
        x.covered_bases_file
        x.covered_bases
    }
}

task x {
    # inputs
    File cw
    String pair_id

    command {
        ...
    }

    runtime {
                     ...
    }

    output {
        File covered_bases_file="${pair_id}.covered_bases.txt"
        Int covered_bases=read_int("${pair_id}.covered_bases.txt")
    }
}

Outputs:
The value of "covered_bases" is "this.covered_bases".

Thanks,
Chunyang