JesAsyncBackendJobExecutionActor failed and didn't catch its exception. : runtime attr validton fail

by network tab inspection (from google developer tools) I saw :

  },
  "submission": "2017-03-30T21:13:32.218Z",
  "status": "Failed",
  "failures": [{
    "message": "JesAsyncBackendJobExecutionActor failed and didn't catch its exception.",
    "causedBy": {
      "message": "Runtime attribute validation failed:\n:\nDisk strings should be of the format 'local-disk SIZE TYPE' or '/mount/point SIZE TYPE'"
    }
  }],

In my WDL, I have :

runtime {
    memory: "${memGB} GB"
    cpu : "1"
    docker: "broadinstitute/lego_plotter"
    disks: "local-disk ${diskGB} HDD"
    }

as the runtime block

and

#runtime parameter for memory/disk
Float memGB
Float diskGB

for the task's relevant input parameters.

here (https://github.com/broadinstitute/wdl/blob/develop/SPEC.md#expressions) I found that float+string=float.... is that relevant information? Should the Float diskGB be a String type? or an Int type?