How do I add the Membership Services tables to an existing SQL Server Express DB?

I am developing an ASP.NET 3.5 app that uses Membership Services. My Windows hosting service only permits a single SQL Server 2005 database so I have used slightly modified versions of the standard Microsoft scripts InstallCommon.sql and InstallMembership.sql (found in the C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727 folder) to add all the relevant tables, etc, to my main application database rather than a default ASPNET.mdf database. The modifications were to replace a couple of references in each script to ASPNET with the name of my existing database. As long as my Membership Provider uses a connection string that points at this online version of my database it works fine. 

So far, so good. My next step was to try and do the same thing to the SQL Server Express version of my database (called COMPDATA2XSQLBE.mdf) installed locally on my PC. However, the InstallCommon.sql script failed with the errors shown below. I suspect that this script (and also InstallMembership. ...