Error - Conversion Of "DBNull" To "Boolean" Is Not Valid

I have a checkbox called Yes on my datagridview. On my datagridview, I have three records and each has the checkbox "Yes" which are unchecked and the values are saved as Null in the table in the database. Now, when I check one of the Yes checkboxes and call the save function. I receive the error - Conversion of "DBNull" to "Boolean" is not valid.

The code is as shown below;

For irow = 0 To DgvReturns.Rows.Count - 1
query = "UPDATE dbo.tblProjectPatients SET Yes ='" & _
CBool(DgvReturns.Rows(irow).Cells(2).Value) & "'where ProjectID = " & _

[code]....

Now, the problem is because there are other two records with values as DBNull on the datagridview i.e.., (it cant update it to the database, hence the unhandledexception).Therefore, can in the update statement state that if the Yes checkbox is not checked to be saved as False to the database.