Field coordinates and moving averages

[Benjamin Beckwith <bnbeckwith@gmail.com>]

I'm trying to create a table of periodic data and calculate a moving
average in an adjacent column.  I was trying to use field coordinates,
but they do not work how I would like them to work.

Here is what I have for a 5-day moving average:

| Daily Data | Moving Average |
|------------+----------------|
|         10 |                |
|         11 |                |
|         12 |                |
|         13 |                |
|         14 |                |
|         14 |                |
|         16 |                |
#+TBLFM: $2=if(@# >=7, vmean(@-5$1..@0$1),string(""));

I expeced the if(@# >=7..) to protect the sub-clauses from executing
unless the predicate is true.  However, that is not the case.  When I
try to execute this formula, I receive the error: "Row descriptor -5
used in line ### leads outside table".

I can see how this is true if the ranges are checked for every entry.
Is there a better way for me to write this column forumla?  Is this a
limitation in the field coordinates?

Thanks,
Ben