c# - dependencyproperty.unsetvalue exception in multibinding with DataGrid -

i have bound datagrid list sysrebardefinitions in view model. @ first okay , data loaded expected, when try edit cell. dependencyproperty.unsetvalue error in converter. can problem?

<datagrid itemssource="{binding path=sysrebardefinitions}">     <datagrid.columns>         <datagridtextcolumn header="weight"                              width="*">             <datagridtextcolumn.binding>                 <multibinding converter="{staticresource rebarlengthanddensitytoweightmulticonverter}"                                      stringformat="{}{0:0.###}">                     <binding path="stocklength"/>                     <binding path="weight"/>                 </multibinding>             </datagridtextcolumn.binding>         </datagridtextcolumn>     </datagrid.columns> </datagrid> 

i have got red arrows question not know why here solution found! check unsetvalue in converter , return.

if (values[0] == dependencyproperty.unsetvalue ||     values[1] == dependencyproperty.unsetvalue) {     return null; }