Yeah.
The only formula to work correctly in this situation is a simple
=AVERAGE(A1:A12)
With a caveat that zero values MUST be shown as zero and not blank
and blank values MUST be shown as blank and not zero.

Month 1. $200 Loss
Month 2. $100 Loss
Month 3. $0 Broke even
Month 4. $200 Profit
Month 5. $400 Profit.
Month 6. ""
Month 7. ""
Month 8. ""
etc.

Regards
Martin