Use of unassigned local variable filterQuery C#

Below is the code where I want to concatenate a string variable with string:

public void formQueryString()
        {
            string filterQuery;
            int chk=0;
            if (rbHomeDelivery.Checked)
            {
                filterQuery = "Home_Delivery like yes";
                chk=1;
            }
            if(rbTakeaway.Checked)
            {
                filterQuery = "Take_Away like yes";
                chk=1;
            }
            if (cbOpenAllDay.Checked && chk==1)
            {
                filterQuery = filterQuery + " or Open_All_Day like yes"; //Line with error
            }
            else
            {
                filterQuery = filterQuery + "Open_All_Day like yes";
            }
        }

It is giving error that "use of unassigned local variable filterQuery ".

Now I don't know why it is giving error.

Please let me know the error

Thanks in advance.