I have a field that calculates feet of garden row based on number of plants and spacing of them.

I can't seem to get the following formula to work:

( no_of_plants * plant_spacing / no_of_plants_wide ) / 12

I keep getting an error that the variable is not declared.  Tried deleting field and re-adding then recreating the formula, no luck.  Also tried rearranging the the fields in the formula.  Keeps saying that the first field name in the formula isn't declared.  Am I doing something wrong?  How do I fix or work around this?