SPSearchServiceApp exception creating defaultAccount credential in Get-TargetResource

Details of the scenario you try and problem that is occurring:
During the last phase of my SP2016 DSC rollout, the SPSearchServiceApp resource is throwing an exception. The exception is:

PowerShell DSC resource MSFT_SPSearchServiceApp failed to execute Test-TargetResource functionality with error message: Exception calling ".ctor" with "2" argument(s): "Cannot process argument because the value of argument "userName" is not valid. Change the value of the "userName" argument and run the operation again."

I traced this down to line 94 in Get-TargetResource (the last line here):

$c = [Microsoft.Office.Server.Search.Administration.SearchContext]::GetContext($serviceApp.Name)
$sc = New-Object -TypeName Microsoft.Office.Server.Search.Administration.Content `
    -ArgumentList $c; 
$dummyPassword = ConvertTo-SecureString -String "-" -AsPlainText -Force 
$defaultAccount = New-Object -TypeName System.Management.Automation.PSCredential `
    -ArgumentList @($sc.DefaultGatheringAccount, $dummyPassword)

My resource block looks like this:

SPSearchServiceApp SearchServiceApp
{
    Name = 'Search Service Application'
    DatabaseName = 'SP_Search'
    DatabaseServer = 'SPSQL'
    ApplicationPool = $serviceAppPoolName
    DefaultContentAccessAccount = $ContentAccessAccount
    CloudIndex = $false
    PsDscRunAsCredential = $SPSetupAccount
    DependsOn = '[WaitForAll]WaitForMainServiceAppPool'
}

I'm going to try to replicate in my Azure lab to see if I get the same results, but wanted to go ahead and post this here to get some eyes on it in case I'm doing something wrong and/or there's an easy fix.

The DSC configuration that is using the resource:

Version of the Operating System and PowerShell the DSC Target Node is running:
WS2012R2; PS 5.1.14409.1005

Version of the DSC module you're using:
1.6.0.0

Start in the SPSearchServiceApp Get-TargetResource function, especially the credential construction at line 94, and reproduce the failure on the stated Windows Server 2012 R2, PowerShell 5.1, and SharePointDsc 1.6.0. Done means Test-TargetResource completes without the invalid userName exception for the provided resource configuration.