DataGrid: "... is not a valid value..."

I have my own derived datagrid. It consists of two columns. Sometimes I get the following error when clicking in the second column: "'200' is not a valid value for 'value'. 'value' should be between 'minimum' and 'maximum'". The only hint I already found out, is, that the number given in the message (in my example the 200) is the column width of my first column. But just like I said, I click in the second column. Any idea?