arcpy.da cursor : iteration not started

I am trying to convert cursors written in 10.0 to use the new arcpy.da module in 10.2.1. If I step through the code in debug mode, it steps into the for loop; however, when I attempt to run the code I get an exception : iteration not started

code for 10.0

for row in updateCursor:
    if row.getValue("STREET_NUMBER_PREFIX") in ['N', 'S', 'W']:
        row.setValue("STREET_DIR_PREFIX",  row.getValue("STREET_NUMBER_PREFIX"))
        row.setValue("STREET_NUMBER_PREFIX", None)

code for 10.2.1

with arcpy.da.UpdateCursor(civicAddress, flds) as rows:
    for row in rows:
        if row[0] in ['N', 'S', 'W']:
            row[1] = row[0]
            row[0] = None

What can I do to solve my problem? I have quite a few code blocks as above that deal with specific logic cases. Many thanks