diff --git a/common/lib/capa/capa/responsetypes.py b/common/lib/capa/capa/responsetypes.py
index fda70c6a55..f4f5d854a9 100644
--- a/common/lib/capa/capa/responsetypes.py
+++ b/common/lib/capa/capa/responsetypes.py
@@ -1869,8 +1869,6 @@ class FormulaResponse(LoncapaResponse):
                 log.debug('formularesponse: error %s in formula' % err)
                 raise StudentInputError("Invalid input: Could not parse '%s' as a formula" %
                                         cgi.escape(given))
-            if numpy.isnan(student_result) or numpy.isinf(student_result):
-                return "incorrect"
             if not compare_with_tolerance(student_result, instructor_result, self.tolerance):
                 return "incorrect"
         return "correct"
diff --git a/common/lib/capa/capa/util.py b/common/lib/capa/capa/util.py
index 9f3e8bd3a0..c219a7b5f6 100644
--- a/common/lib/capa/capa/util.py
+++ b/common/lib/capa/capa/util.py
@@ -1,4 +1,5 @@
 from .calc import evaluator, UndefinedVariable
+from cmath import isinf
 
 #-----------------------------------------------------------------------------
 #
@@ -20,8 +21,11 @@ def compare_with_tolerance(v1, v2, tol):
         tolerance = tolerance_rel * max(abs(v1), abs(v2))
     else:
         tolerance = evaluator(dict(), dict(), tol)
-    return abs(v1 - v2) <= tolerance
 
+    if isinf(v1) or isinf(v2):
+        return v1 == v2 # because the other numerical comparison does not work with infinities
+    else:
+        return abs(v1 - v2) <= tolerance
 
 def contextualize_text(text, context):  # private
     ''' Takes a string with variables. E.g. $a+$b.