Fairly simple calculation script and subsequent error

Hi You fantastic debugger/educators!

Here is my script:

var vsupplycfm = Number(this.getField("SupplyCFMcalc").valueAsString);

var vexhaustcfm = Number(this.getField("iftheninflow").valueAsString);

var vnozzle = Number(this.getField("Nozzledropdown").valueAsString);

event.value = (13500*vnozzle) / (vsupplycfm+vexhaustcfm);

its a fairly simple calculation I want to run at the end.  And here is what the javascript debugger says:

Exception in line 3 of function top_level, script Field:Calculate

TypeError: this.getField(...) is null

3:Field:CalculateException in line 1 of function top_level, script Field:Calculate

TypeError: this.getField(...) is null

1:Field:Calculate

But the other fields are working and have numbers in them and are not null. I checked spelling , capitals etc seem correct.