Problem accessing profile information (Value cannot be null. Parameter name: type)

I am having a problem accessing profile information in a web project. I have set up a custom profile provider:
 
    <profile defaultProvider="defaultProfile" enabled="True">
      <providers>
        <clear />
        <add name="defaultProfile" type="System.Web.Profile.SqlProfileProvider" connectionStringName="ConnectionString"/>
      </providers>
      <properties>
        <add name="Surname" type="System.String"/>
      </properties>
    </profile>
 
This works on my development machine, and used to work on the production servers, but now every time I ...