DBNull.Value

Some cells of one field are "EMPTY". and this field has Decimal(5, 2)  data type. When I do the update, I tried to use 
dt.Rows[row.DataItemIndex]["FIELDNAME"] = DBNull.Value;
and it has an error like 
Error converting data type varchar to numeric. even I did not do the real updating (I just click the edit and did nothing and then click update). The error message is above. How can I fix it?

Thanks.
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