Problem Passing A Variable To A Stored Procedure

I've got stored procedure:

  ALTER PROCEDURE [dbo].[dropmyValue]
(
@dropVal Char OUTPUT)
AS
EXECUTE('ALTER TABLE [dbo].[tbl1] DROP CONSTRAINT ' + @dropVal)
  That gets it's value from a GridView.SelectedValue:

Protected Sub GridView1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        If GridView1.SelectedValue.ToString <> "" Then
            Dim cs As String = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
            Using con As New SqlConnection(cs)
                con.Open()
                Dim cmd As New SqlCommand
                cmd.Connection = con
                cmd.CommandType = CommandType.StoredProcedure
                cmd.CommandText = "dropmyValue"
                cmd.Parameters.Add("dropVal", SqlDbType.Char)
                cmd.Parameters("dropVal").Direction = ParameterDirection.InputOutput
                cmd.Parameters("dropVal").Value = "DF_" + GridView1.SelectedValue
                Label2.Text = cmd.Parameters("dropVal").Value
                cmd.ExecuteNonQuery()
            End Using
            GridView1.DataBind()
        End If
    End Sub 

Label2.text shows that @dropVal is "DF_xxxxxxxx" and is the name of the Constaint to be dropped (when I comment out "cmd.ExecuteNonQuery" and run it), but the error I get is that

" 'D' is not a Constraint ". I don't know if this is a sqldbtype problem, but I've tried different ones and evidently only the first character "D" is getting read, or passed to the stored procedure.

Any help would be appreciated.

Steve