Web service returns null ProviderUserKey in MembershipUser

I have a custom membership provider that I am calling from a web service. The web service is used by a client application that will need to authenicate the user and will allow the user to upload data to a database. The web service works with the membership provider and returns a membershipuser. The MembershipUser has the ProviderUserKey and UserName but once the web service returns the MembershipUser object to my client application, the ProviderUserKey is null. Any ideas why this is?
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