diff --git a/tests/pipeline/test_classifier.py b/tests/pipeline/test_classifier.py
index de111e97..aba52225 100644
--- a/tests/pipeline/test_classifier.py
+++ b/tests/pipeline/test_classifier.py
@@ -8,7 +8,6 @@ from zipline.pipeline import Classifier
 from zipline.testing import parameter_space
 from zipline.utils.numpy_utils import (
     categorical_dtype,
-    coerce_to_dtype,
     int64_dtype,
 )
 
@@ -162,7 +161,8 @@ class ClassifierTestCase(BasePipelineTestCase):
         dtype_=[int64_dtype, categorical_dtype],
     )
     def test_disallow_comparison_to_missing_value(self, missing, dtype_):
-        missing = coerce_to_dtype(dtype_, missing)
+        if dtype_ == categorical_dtype:
+            missing = str(missing)
 
         class C(Classifier):
             dtype = dtype_
@@ -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
 
diff --git a/tests/pipeline/test_factor.py b/tests/pipeline/test_factor.py
index c5873c1f..fc4eef69 100644
--- a/tests/pipeline/test_factor.py
+++ b/tests/pipeline/test_factor.py
@@ -23,6 +23,7 @@ from numpy import (
 from numpy.random import randn, seed
 
 from zipline.errors import UnknownRankMethod
+from zipline.lib.labelarray import LabelArray
 from zipline.lib.rank import masked_rankdata_2d
 from zipline.lib.normalize import naive_grouped_rowwise_apply as grouped_apply
 from zipline.pipeline import Classifier, Factor, Filter, TermGraph
@@ -38,6 +39,7 @@ from zipline.testing import (
 )
 from zipline.utils.functional import dzip_exact
 from zipline.utils.numpy_utils import (
+    categorical_dtype,
     datetime64ns_dtype,
     float64_dtype,
     int64_dtype,
@@ -442,6 +444,7 @@ class FactorTestCase(BasePipelineTestCase):
         f = self.f
         m = Mask()
         c = C()
+        str_c = C(dtype=categorical_dtype, missing_value=None)
 
         factor_data = array(
             [[1.0, 2.0, 3.0, 4.0],
@@ -463,12 +466,18 @@ class FactorTestCase(BasePipelineTestCase):
              [1, 1, 2, 2]],
             dtype=int64_dtype,
         )
+        string_classifier_data = LabelArray(
+            classifier_data.astype(str).astype(object),
+            missing_value=None,
+        )
 
         terms = {
             'vanilla': f.demean(),
             'masked': f.demean(mask=m),
             'grouped': f.demean(groupby=c),
+            'grouped_str': f.demean(groupby=str_c),
             'grouped_masked': f.demean(mask=m, groupby=c),
+            'grouped_masked_str': f.demean(mask=m, groupby=str_c),
         }
         expected = {
             'vanilla': array(
@@ -496,6 +505,9 @@ class FactorTestCase(BasePipelineTestCase):
                  [-0.500,  0.500,  0.000,    nan]]
             )
         }
+        # Changing the classifier dtype shouldn't affect anything.
+        expected['grouped_str'] = expected['grouped']
+        expected['grouped_masked_str'] = expected['grouped_masked']
 
         graph = TermGraph(terms)
         results = self.run_graph(
@@ -503,6 +515,7 @@ class FactorTestCase(BasePipelineTestCase):
             initial_workspace={
                 f: factor_data,
                 c: classifier_data,
+                str_c: string_classifier_data,
                 m: filter_data,
             },
             mask=self.build_mask(self.ones_mask(shape=factor_data.shape)),
diff --git a/tests/pipeline/test_term.py b/tests/pipeline/test_term.py
index 502d6e48..4b7bf0de 100644
--- a/tests/pipeline/test_term.py
+++ b/tests/pipeline/test_term.py
@@ -602,3 +602,26 @@ class SubDataSetTestCase(TestCase):
         with self.assertRaises(ValueError) as e:
             SomeClassifier()
         self.assertEqual(str(e.exception), expected_error)
+
+    def test_unreasonable_missing_values(self):
+
+        for base_type, dtype_, bad_mv in ((Factor, float64_dtype, 'ayy'),
+                                          (Filter, bool_dtype, 'lmao'),
+                                          (Classifier, int64_dtype, 'lolwut'),
+                                          (Classifier, categorical_dtype, 7)):
+            class SomeTerm(base_type):
+                inputs = ()
+                window_length = 0
+                missing_value = bad_mv
+                dtype = dtype_
+
+            with self.assertRaises(TypeError) as e:
+                SomeTerm()
+
+            prefix = (
+                "^Missing value {mv!r} is not a valid choice "
+                "for term SomeTerm with dtype {dtype}.\n\n"
+                "Coercion attempt failed with:"
+            ).format(mv=bad_mv, dtype=dtype_)
+
+            self.assertRegexpMatches(str(e.exception), prefix)
diff --git a/zipline/lib/labelarray.py b/zipline/lib/labelarray.py
index b707f0cd..a3ef5027 100644
--- a/zipline/lib/labelarray.py
+++ b/zipline/lib/labelarray.py
@@ -211,6 +211,10 @@ class LabelArray(ndarray):
         # This is a property because it should be immutable.
         return self._missing_value
 
+    @property
+    def missing_value_code(self):
+        return self.reverse_categories[self.missing_value]
+
     def has_label(self, value):
         return value in self.reverse_categories
 
diff --git a/zipline/pipeline/classifiers/classifier.py b/zipline/pipeline/classifiers/classifier.py
index 05a99593..6a64e106 100644
--- a/zipline/pipeline/classifiers/classifier.py
+++ b/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__,
                 )
             )
 
diff --git a/zipline/pipeline/factors/factor.py b/zipline/pipeline/factors/factor.py
index a76002a9..6a0b0b08 100644
--- a/zipline/pipeline/factors/factor.py
+++ b/zipline/pipeline/factors/factor.py
@@ -47,6 +47,7 @@ from zipline.utils.input_validation import expect_types
 from zipline.utils.math_utils import nanmean, nanstd
 from zipline.utils.numpy_utils import (
     bool_dtype,
+    categorical_dtype,
     coerce_to_dtype,
     datetime64ns_dtype,
     float64_dtype,
@@ -939,15 +940,26 @@ class GroupedRowTransform(Factor):
 
     def _compute(self, arrays, dates, assets, mask):
         data = arrays[0]
-        null_group_value = self.inputs[1].missing_value
-        group_labels = where(
-            mask,
-            arrays[1],
-            null_group_value,
-        )
+        groupby_expr = self.inputs[1]
+        if groupby_expr.dtype == int64_dtype:
+            group_labels = arrays[1]
+            null_label = self.inputs[1].missing_value
+        elif groupby_expr.dtype == categorical_dtype:
+            # Coerce our LabelArray into an isomorphic array of ints.  This is
+            # necessary because np.where doesn't know about LabelArrays or the
+            # void dtype.
+            group_labels = arrays[1].as_int_array()
+            null_label = arrays[1].missing_value_code
+        else:
+            raise TypeError(
+                "Unexpected groupby dtype: %s." % groupby_expr.dtype
+            )
+
+        # Make a copy with the null code written to masked locations.
+        group_labels = where(mask, group_labels, null_label)
 
         return where(
-            group_labels != null_group_value,
+            group_labels != null_label,
             naive_grouped_rowwise_apply(
                 data=data,
                 group_labels=group_labels,
diff --git a/zipline/pipeline/term.py b/zipline/pipeline/term.py
index 1bffc3e7..4b04d465 100644
--- a/zipline/pipeline/term.py
+++ b/zipline/pipeline/term.py
@@ -4,7 +4,7 @@ Base class for Filters, Factors and Classifiers
 from abc import ABCMeta, abstractproperty
 from weakref import WeakValueDictionary
 
-from numpy import dtype as dtype_class, ndarray
+from numpy import array, dtype as dtype_class, ndarray
 from six import with_metaclass
 from zipline.errors import (
     DTypeNotSpecified,
@@ -16,10 +16,12 @@ from zipline.errors import (
     WindowLengthNotSpecified,
 )
 from zipline.lib.adjusted_array import can_represent_dtype
+from zipline.lib.labelarray import LabelArray
 from zipline.utils.input_validation import expect_types
 from zipline.utils.memoize import lazyval
 from zipline.utils.numpy_utils import (
     bool_dtype,
+    categorical_dtype,
     default_missing_value_for_dtype,
 )
 from zipline.utils.sentinel import sentinel
@@ -177,6 +179,7 @@ class Term(with_metaclass(ABCMeta, object)):
         """
         if dtype is NotSpecified:
             raise DTypeNotSpecified(termname=termname)
+
         try:
             dtype = dtype_class(dtype)
         except TypeError:
@@ -188,6 +191,31 @@ class Term(with_metaclass(ABCMeta, object)):
         if missing_value is NotSpecified:
             missing_value = default_missing_value_for_dtype(dtype)
 
+        try:
+            if (dtype == categorical_dtype):
+                # This check is necessary because we use object dtype for
+                # categoricals, and numpy will allow us to promote numerical
+                # values to object even though we don't support them.
+                _assert_valid_categorical_missing_value(missing_value)
+
+            # For any other type, we can check if the missing_value is safe by
+            # making an array of that value and trying to safely convert it to
+            # the desired type.
+            # 'same_kind' allows casting between things like float32 and
+            # float64, but not str and int.
+            array([missing_value]).astype(dtype=dtype, casting='same_kind')
+        except TypeError as e:
+            raise TypeError(
+                "Missing value {value!r} is not a valid choice "
+                "for term {termname} with dtype {dtype}.\n\n"
+                "Coercion attempt failed with: {error}".format(
+                    termname=termname,
+                    value=missing_value,
+                    dtype=dtype,
+                    error=e,
+                )
+            )
+
         return dtype, missing_value
 
     def __init__(self, *args, **kwargs):
@@ -498,3 +526,20 @@ class ComputableTerm(Term):
             inputs=self.inputs,
             window_length=self.window_length,
         )
+
+
+def _assert_valid_categorical_missing_value(value):
+    """
+    Check that value is a valid categorical missing_value.
+
+    Raises a TypeError if the value is cannot be used as the missing_value for
+    a categorical_dtype Term.
+    """
+    label_types = LabelArray.SUPPORTED_SCALAR_TYPES
+    if not isinstance(value, label_types):
+        raise TypeError(
+            "Categorical terms must have missing values of type "
+            "{types}.".format(
+                types=' or '.join([t.__name__ for t in label_types]),
+            )
+        )