Variable stored as NoneType in Python with updateCursor

Question

SOLVED: although the field was not nullable (no null values), a glitch created one in the heading field, so the variable was stored as a null value and thus the NoneType.

I am using updateCursor to subtract a previous value (last direction) from a current value (direction) to populate a new field (change in direction). I borrowed and modified code from the forums (thanks anonymous contributors!) so while I understand the conditionals and what they all do, I did not write this myself.

Code:

import arcpy
fc = r'...'
rows = arcpy.UpdateCursor(fc)
firstTime = True
for row in rows:
    if firstTime:
        previous = row.heading
        firstTime = False
    else:
        row.angle = row.heading - previous
        rows.updateRow(row)
        previous = row.heading
del row, rows

The error I get is:

Runtime error <type 'exceptions.TypeError'>: unsupported operand type(s) for -: 'int' and 'NoneType'

From what I understand because I have no return function after I define my variable previous, it gets erased and the variable is read as NoneType. However with multiple tries I've figured out that the return command only works in def function() codeblocks.
Is there an equivalent to return for updateCursor?

UPDATE: this similar Codeblock produces the same results.

import arcpy
fc = r'...'
rows = arcpy.UpdateCursor(fc)
count = 0
for row in rows:
    if count != 0:
        count = count + 1
        row.angle = row.heading - previous
        rows.updateRow(row)
        previous = row.heading
    else:
        count = 1
        previous = row.heading
del row, rows