diff --git a/common/lib/calc/calc.py b/common/lib/calc/calc.py
index 2271676f34..2f33b66bfd 100644
--- a/common/lib/calc/calc.py
+++ b/common/lib/calc/calc.py
@@ -144,6 +144,8 @@ def evaluator(variables, functions, string, cs=False):
         return x
 
     def parallel(x):  # Parallel resistors [ 1 2 ] => 2/3
+        # convert from pyparsing.ParseResults, which doesn't support '0 in x'
+        x = list(x)
         if len(x) == 1:
             return x[0]
         if 0 in x:
diff --git a/common/lib/calc/tests/test_calc.py b/common/lib/calc/tests/test_calc.py
index cd28801d83..58d0860af6 100644
--- a/common/lib/calc/tests/test_calc.py
+++ b/common/lib/calc/tests/test_calc.py
@@ -92,6 +92,10 @@ class EvaluatorTest(unittest.TestCase):
         """
         self.assertRaises(ZeroDivisionError, calc.evaluator,
                           {}, {}, '1/0')
+        self.assertRaises(ZeroDivisionError, calc.evaluator,
+                          {}, {}, '1/0.0')
+        self.assertRaises(ZeroDivisionError, calc.evaluator,
+                          {'x': 0.0}, {}, '1/x')
 
     def test_parallel_resistors(self):
         """
@@ -107,12 +111,13 @@ class EvaluatorTest(unittest.TestCase):
         self.assertEqual(calc.evaluator({}, {}, '1||1||2'), 0.4)
         self.assertEqual(calc.evaluator({}, {}, "j||1"), 0.5 + 0.5j)
 
-    def test_parallel_resistors_zero_error(self):
+    def test_parallel_resistors_with_zero(self):
         """
         Check the behavior of the || operator with 0
         """
-        self.assertRaises(ZeroDivisionError, calc.evaluator,
-                          {}, {}, '0||1')
+        self.assertTrue(numpy.isnan(calc.evaluator({}, {}, '0||1')))
+        self.assertTrue(numpy.isnan(calc.evaluator({}, {}, '0.0||1')))
+        self.assertTrue(numpy.isnan(calc.evaluator({'x': 0.0}, {}, 'x||1')))
 
     def assert_function_values(self, fname, ins, outs, tolerance=1e-3):
         """
diff --git a/common/lib/capa/capa/tests/test_responsetypes.py b/common/lib/capa/capa/tests/test_responsetypes.py
index 77cd547e55..780c475b09 100644
--- a/common/lib/capa/capa/tests/test_responsetypes.py
+++ b/common/lib/capa/capa/tests/test_responsetypes.py
@@ -10,7 +10,6 @@ import random
 import unittest
 import textwrap
 import mock
-import textwrap
 
 from . import new_loncapa_problem, test_system