Argument 'Expression' cannot be converted to type 'DBNull'

[Sep410]

Hello,
Here is the code that I have :

 If Not Val(dgFamilyMember.Rows(i).Cells(15).Value).Equals(Val(publicFamilyMemebrDS.Tables(0).Rows(i).Item(15).ToString)) Then
                    StrFMCommet &= " FM Group: " & publicFamilyMemebrDS.Tables(0).Rows(i).Item(14).ToString & ""
                    StrFMCommet &= " Changed to " & dgFamilyMember.Rows(i).Cells(14).Value & "    ***     "
                    If flagClientDetailFMUpdated = False Then UpdateClientDetails("FM")
                End If
 

When user removes a field's value from the datagrid and press save button. It will be check to see if data had changed. If it is changed the comment record will be added to database.
Argument 'Expression' cannot be converted to type 'DBNull'.
I get this error and I don’t know how should I handle it I have the same code for 50 other fields.

[Plater]

Are you sure that is the correct line of code as pointed to by the error message?
I don't see any explicit usage of DBNull?