Up gaps on 1st of the month

Today is the first trading day of February.  Further to the first of the month bias post, we are now trading well above yesterdays close.  Below is a equity curve of a strategy that shorts any up ES gap on the first trading day of the month.  The strategy exits at gap fill or end of the day.

As you can see this has been a losing strategy historically.  Only 49% of gaps have filled, which is well below historical norms.