When "prompt" isn't defined in the runspace the fallback to DefaultPrompt doesn't work because we aren't catching the CommandNotFoundException https://github.com/PowerShell/PowerShellEditorServices/blob/b433b0f2d1b0e9b93466b5e307a202bfe290c841/src/PowerShellEditorServices/Services/PowerShell/Host/PsesInternalHost.cs#L662-L664