How do I include empty value cells in an average function in Google Spreadsheet?

I’m in the process of preparing accounts for court and the court wants me to average all the irregular expenses monthly. When I do a simple average in Google docs it only takes the month that has a value in it and ignores all the empty cells in the average solution which skews the results. I don’t want to add zeros to all the empty cells. I want each calculation to be averaged over 12.

Anyone know how I can achieve this?