General summary of the issue

When mock function fails parameter validation, the original function is executed with no warnings/errors reported.

Describe your environment

Pester version : 5.3.0 F:\Users\bozho.powershell\modules\Pester\5.3.0\Pester.psm1
PowerShell version : 5.1.19041.1645
OS version : Microsoft Windows NT 10.0.19044.0

Steps to reproduce

I have a module function that looks something like this:

function Get-SomeRestResource {
    [CmdletBinding(PositionalBinding = $false)]
    param(
        [parameter()]
        [ValidateNotNull()]
        [string] $Server = $script:DefaultServer
    )

    try {
        $headers = @{
            Authorization = "...."
            "Content-Type" = "application/json"
        }

        $response = Invoke-RestMethod -Uri "https://$Server/api/some/resource" -Method Get -Headers $headers
    }
    catch {
        ....
    }
}

$script:DefaultServer is a module variable.

I am mocking both Get-SomeRestResource and Invoke-RestMethod. I am mocking Invoke-RestMethod. Mocked Invoke-RestMethod simply returns a constant JSON object (doesn't reference $Server parameter at all).

I have a test where I simply execute Get-SomeRestResource with no parameters, so $Server is supposed to pick up the default server string.

I had a bug where $script:DefaultServer module variable was not defined (I forgot to initialise it in the module), which resulted in $Server parameter being an empty string.

This in turn resulted in the original Get-SomeRestResource being executed - I've confirmed that both using VS Code's test debugging and the fact that the original Invoke-RestMethod was complaining about invalid URL (https:///api/some/resource).

This in turn resulted in the original Invoke-RestMethod being executed - I've confirmed that both using VS Code's test debugging and the fact that the original Invoke-RestMethod was complaining about invalid URL (https:///api/some/resource).

The only change I had to make is to actually declare $script:DefaultServer module variable, with no other changes to the module or test code and everything worked as expected.

Expected Behavior

Parameter validation error being raised.