Procedure or function VendorCoverageInsert has too many arguments specified.

Can anyone tell me why I am getting this error?  Am I not referencing the checkbox properly?  I would like the user to select as many zip codes from the gridview, click the submit button and append all selected records to the table.  Thanks.

Code behind called when user clicks the button

Protected Sub btnAddSelectedCoverage_Click(sender As Object, e As EventArgs) Handles btnAddSelectedCoverage.Click
        Dim cnn As New System.Data.SqlClient.SqlConnection(ConfigurationManager.ConnectionStrings("myprovalConnectionString").ConnectionString)
        Try
            Dim cmd As New System.Data.SqlClient.SqlCommand
            cmd.Connection = cnn
            cmd.CommandType = Data.CommandType.StoredProcedure
            cmd.CommandText = "dbo.VendorCoverageInsert"
            For Each gvRow As GridViewRow In gdvZipList.Rows
                Dim chkBox As CheckBox = CType(gvRow.FindControl("RowLevelCheckBox"), CheckBox)
                If chkBox.Checked Then
                    cmd.Parameters.Add("@VendorId", Data.SqlDbType.VarChar).Value = Session("VendorId").ToString
                    cmd.Parameters.Add("@UserId", Data.SqlDbType.VarChar).Value = Session("VendorId").ToString
                    cmd.Parameters.Add("@Coverage", Data.SqlDbType.Char).Value = chkBox.ToString
                End If
            Next
            Dim dt As New System.Data.DataTable
            cnn.Open()
            cmd.ExecuteScalar()
            Dim sRedirect As String = "VendorAddEditCoverage.aspx?VID=" + Session("VendorId").ToString
            Response.Redirect(sRedirect)
        Finally
            cnn.Close()
        End Try
    End Sub

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Stored Procedure called by the code behind

ALTER PROCEDURE dbo.VendorCoverageInsert
	(
	@UserId VarChar(50),
	@VendorId Varchar(50),
	@Coverage char(5)
	)
AS


BEGIN
	INSERT INTO VendorCoverageArea
	                         (VendorId, ZipCode, CreatedUser, ModifiedUser)
	VALUES        (@VendorId,@Coverage,@UserId,@UserId)
	               
	END

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Gridview

<asp:GridView ID="gdvZipList" runat="server" AutoGenerateColumns="False" 
                            CssClass="mGrid" Width="257px">
                            <Columns>
                                <asp:TemplateField HeaderText="Select">
                               
            <EditItemTemplate>
                                        <asp:CheckBox ID="RowLevelCheckBox" runat="server" Checked='<%# Bind("ZipCode") %>' />
                                    </EditItemTemplate>
                                    <ItemTemplate>
                                        <asp:CheckBox ID="RowLevelCheckBox" runat="server" Checked='<%# Bind("ZipCode") %>' 
                                            Enabled="false" />
                                    </ItemTemplate>
                                </asp:TemplateField>
                                <asp:BoundField DataField="ZipCode" HeaderText="Zip Code" />
                            </Columns>
                             
                        </asp:GridView>

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