Can't execute a simple psycopg2 script twice when running from ArcGIS Desktop as a script tool

I’m building a geoprocessing service that queries an SDE database (running on postgres) and returns results. I do a lot of development by calling the script from a toolbox in ArcGIS desktop 10.3.

For some reason when I call the script below in ArcGIS desktop, it executes successfully the first time, then fails every time after that with ‘NoneType’ object is not callable
on line 12 as I try to iterate over inAllCornerPointsList. If I close ArcMap and call it, it works the first time, then fails.

import psycopg2

conn = psycopg2.connect(host='localhost', database ='sde', user='sde', password='somepassword')
curs = conn.cursor()

sqlStatement = """SELECT * FROM sde.gly_ula_corners"""

curs.execute(sqlStatement)
inAllCornerPointsList = curs.fetchall()

for cornerPoint in inAllCornerPointsList:
    print cornerPoint

curs.close()
conn.close()

print 'Complete'

To further confuse things, if I select * from a different table (sde.countyboundaries), my script executes successfully every time.

The geometry type for sde.gly_ula_corners is shape st_point. The geometry type for sde.countyboundaries is shape st_geometry. This is the only difference that jumps out at me right away– one is a point fc, and one is a polygon.

Any idea what might be causing this? Is there some weird memory environment at work? Calling the script multiple times from the command line or pythonwin does not fail.