Unable to update listitem external data column

I am trying to update listitem column which is based on external data type (BCS) from client object model (code snippet shown below).

listItem["Supplier"] = "00000030";
listItem.Update();
ctx.ExecuteQuery();


I don't get any errors but when I view item properties, the column is empty. Do I need to execute "Select External Item(s)" which verifies the value against the data source?