BUG: Fix bad error message for element_of.

It referred to the wrong method name (`is_element`).

tests/pipeline/test_classifier.py

@@ -434,7 +434,7 @@ class ClassifierTestCase(BasePipelineTestCase):
             errmsg = str(e.exception)
             expected = (
                 "Found self.missing_value ('not in the array') in choices"
-                " supplied to C.is_element().\n"
+                " supplied to C.element_of().\n"
                 "Missing values have NaN semantics, so the requested"
                 " comparison would always produce False.\n"
                 "Use the isnull() method to check for missing values.\n"
@@ -447,7 +447,7 @@ class ClassifierTestCase(BasePipelineTestCase):
 
         class C(Classifier):
             dtype = dtype_
-            missing_value = ''
+            missing_value = dtype.type('1')
             inputs = ()
             window_length = 0
 

zipline/pipeline/classifiers/classifier.py

@@ -246,7 +246,7 @@ class Classifier(RestrictedDTypeMixin, ComputableTerm):
         if self.missing_value in choices:
             raise ValueError(
                 "Found self.missing_value ({mv!r}) in choices supplied to"
-                " {typename}.is_element().\n"
+                " {typename}.{meth_name}().\n"
                 "Missing values have NaN semantics, so the"
                 " requested comparison would always produce False.\n"
                 "Use the isnull() method to check for missing values.\n"
@@ -254,6 +254,7 @@ class Classifier(RestrictedDTypeMixin, ComputableTerm):
                     mv=self.missing_value,
                     typename=(type(self).__name__),
                     choices=sorted(choices),
+                    meth_name=self.element_of.__name__,
                 )
             )