I am adjusting the UserAdmin.aspx layout to use my Master Page. When I do this, selecting a user from the user list causes an error:

System.FormatException was unhandled by user code
Message="Input string was not in a correct format."
Source="mscorlib"
StackTrace:
at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
at System.String.System.IConvertible.ToInt32(IFormatProvider provider)
at System.Convert.ToInt32(Object value)
at OakLeaf.MM.Main.mmType.ConvertToDbType(Object objectValue, DbType dbType)
at OakLeaf.MM.Main.Security.mmUser.LoadRow(Object pk)
at SamaanSystems.IAS.Web.Admin.UserAdmin.LoadUser(Object Pk) in C:\Development\Mere Mortals .NET Framework 2008\SamaanSystems\IAS.Web\Admin\UserAdmin.aspx.cs:line 79
at SamaanSystems.IAS.Web.Admin.UserAdmin.lstUsers_SelectedIndexChanged(Object sender, EventArgs e) in C:\Development\Mere Mortals .NET Framework 2008\SamaanSystems\IAS.Web\Admin\UserAdmin.aspx.cs:line 214
at System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e)
at System.Web.UI.WebControls.ListBox.RaisePostDataChangedEvent()
at System.Web.UI.WebControls.ListBox.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent()
at System.Web.UI.Page.RaiseChangedEvents()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
InnerException:

If I take out my master page an dgo back to the default it works. What is it that I am missing?