How do I use Add-Type to load Microsoft.Web.Deployment?

I am writing some PowerShell scripts that use the MSDeploy API. I can load the assembly using

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Web.Deployment")

The location is in the GAC:

PS > [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Web.Deployment") | fl Location

Location : C:\Windows\assembly\GAC_MSIL\Microsoft.Web.Deployment\7.1.0.0__31bf3856ad364e35\Microsoft.Web.Deployment.dll

However, I am not able to load the assembly using Add-Type. I get an error saying the assembly cannot be found and that one or more assemblies are missing.

PS > Add-Type -AssemblyName Microsoft.Web.Deployment
Add-Type : Cannot add type. The assembly 'Microsoft.Web.Deployment' could not be found.
At line:1 char:9
+ Add-Type <<<<  -AssemblyName Microsoft.Web.Deployment
    + CategoryInfo          : ObjectNotFound: (Microsoft.Web.Deployment:String) [Add-Type], Exception
    + FullyQualifiedErrorId : ASSEMBLY_NOT_FOUND,Microsoft.PowerShell.Commands.AddTypeCommand

Add-Type : Cannot add type. One or more required assemblies are missing.
At line:1 char:9
+ Add-Type <<<<  -AssemblyName Microsoft.Web.Deployment
    + CategoryInfo          : InvalidData: (:) [Add-Type], InvalidOperationException
    + FullyQualifiedErrorId : ASSEMBLY_LOAD_ERRORS,Microsoft.PowerShell.Commands.AddTypeCommand

How can I use Add-Type to load Microsoft.Web.Deployment?