Invoke-WebRequest doesn't work until I call WebClient.DownloadString - powershell

Question

I'm using powershell v5 to call an internal API using TLS1.2 with a self-signed cert. When I call the api I always get Invoke-WebRequest : The underlying connection was closed: An unexpected error occurred on a send.
E.g.:
PS> [System.Net.ServicePointManager]::ServerCertificateValidationCallback = {$true}
PS> [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
PS> $url = "https://someinternalserver/blah"
PS> $response = Invoke-WebRequest $url
Invoke-WebRequest : The underlying connection was closed: An unexpected error occurred on a send.
At line:1 char:1
(I've looked in the error object but I don't see anything helpful.)
However if I call the same URL using an instance of WebClient, then the call using WebClient AND all subsequent powershell calls works fine:
PS> $webClient = New-Object System.Net.WebClient
PS> $str = $webClient.DownloadString($url)
PS> Write-Host $str
body of request
PS> $response = Invoke-WebRequest $url
PS> Write-Host $response.Content
body or request
I'm not sure what's going on, but I suspect it has something to do with the self-signed cert, or the crypto. Here's what chrome says about the crypto:
I've used powershell to call APIs with self-signed certs before but never had these kind of issues.
Resolution: I'd like to call the API without first using WebClient.
Thanks.

Answer 1

So dug into the error more and found this in an inner exception:
There is no Runspace available to run scripts in this thread.
You can provide one in the DefaultRunspace property of the System.Management.Automation.Runspaces.Runspace type.
The script block you attempted to invoke was: $true
Which led me here:
Powershell 3.0 Invoke-WebRequest HTTPS Fails on All Requests
Which led me here:
https://stackoverflow.com/a/15841856/6311875
Using that code instead of the {$true} did the trick.
So, this further reinforces the idea that all questions are already answered on SO, you just have to look hard enough.