Handling / Converting SQL 'uniqueidentifier'

my environment: WXP pro, VisualStudio 2005, SQL express Adv. with SP1

When running below code I get following error on statement Nr:14.

Error:
"Implicit conversion from data type sql_variant to uniqueidentifier is not allowed. Use the CONVERT function to run this query."

What would I like to achieve:
In 'mr.memberid' (9) I correctly get the PrimaryKey as 'uniqueidentifier'. In another .aspx.vb file I later use 'Session("MemberGuid")' to fill correctly a GridView based on this value!

Now, on statements (11 - 14) I would like to insert a record where field 'Member' ('Type' is also 'uniqueidentifier') should be filled using this 'uniquidentifier, but as said, I get above msg.

When using (11, no String in 'mr.member' !):
'SqlDSEnrolled.InsertParameters("Member").DefaultValue = mr.memberid' I get error msg: "Value of type 'System.Guid' cannot be converted to 'String'."

Can anybody help me on how to achieve this Conversion correctly and where to do the conversion? Thanks for your time
ed1 Protected Sub EnrollButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
2 If Page.User.Identity.IsAuthenticated Then
3
4 Dim user As MembershipUser = Membership.GetUser()
5
6 Dim da As New DataSetTableAdapters.MemberInfoTableAdapter
7 Dim dt As DataSet.MemberInfoDataTable = da.GetMember(CType(user.ProviderUserKey, Guid))
8 Dim mr As DataSet.MemberInfoRow = dt(0)
9 Session("MemberGuid") = mr.memberid
10
11 SqlDSEnrolled.InsertParameters("Member").DefaultValue = mr.memberid.ToString
12 SqlDSEnrolled.InsertParameters("Event").DefaultValue = CStr(EventID)
13 'SqlDSEnrolled.InsertParameters("HcIndex").DefaultValue = "11.5"
14 SqlDSEnrolled.Insert()
15
16 Response.Redirect("Enroll_Event.aspx")
17 Else
18 Response.Redirect("Login.aspx")
19 End If
20 End Sub