diff --git a/django/db/models/fields/related_lookups.py b/django/db/models/fields/related_lookups.py
index 96eb8c8776..706e37a6bd 100644
--- a/django/db/models/fields/related_lookups.py
+++ b/django/db/models/fields/related_lookups.py
@@ -19,6 +19,9 @@ class MultiColSource:
         return self.__class__(relabels.get(self.alias, self.alias),
                               self.targets, self.sources, self.field)
 
+    def get_lookup(self, lookup):
+        return self.output_field.get_lookup(lookup)
+
 
 def get_normalized_value(value, lhs):
     from django.db.models import Model
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index bdec204351..ccbbe0cd5f 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -964,19 +964,9 @@ class Query:
     def as_sql(self, compiler, connection):
         return self.get_compiler(connection=connection).as_sql()
 
-    def prepare_lookup_value(self, value, lookups, can_reuse, allow_joins=True):
-        # Default lookup if none given is exact.
+    def resolve_lookup_value(self, value, can_reuse, allow_joins):
         used_joins = set()
-        if len(lookups) == 0:
-            lookups = ['exact']
-        # Interpret '__exact=None' as the sql 'is NULL'; otherwise, reject all
-        # uses of None as a query value.
-        if value is None:
-            if lookups[-1] not in ('exact', 'iexact'):
-                raise ValueError("Cannot use None as a query value")
-            lookups[-1] = 'isnull'
-            return True, lookups, used_joins
-        elif hasattr(value, 'resolve_expression'):
+        if hasattr(value, 'resolve_expression'):
             pre_joins = self.alias_refcount.copy()
             value = value.resolve_expression(self, reuse=can_reuse, allow_joins=allow_joins)
             used_joins = {k for k, v in self.alias_refcount.items() if v > pre_joins.get(k, 0)}
@@ -993,15 +983,7 @@ class Query:
                     # The used_joins for a tuple of expressions is the union of
                     # the used_joins for the individual expressions.
                     used_joins.update(k for k, v in self.alias_refcount.items() if v > pre_joins.get(k, 0))
-        # For Oracle '' is equivalent to null. The check needs to be done
-        # at this stage because join promotion can't be done at compiler
-        # stage. Using DEFAULT_DB_ALIAS isn't nice, but it is the best we
-        # can do here. Similar thing is done in is_nullable(), too.
-        if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and
-                lookups[-1] == 'exact' and value == ''):
-            value = True
-            lookups[-1] = 'isnull'
-        return value, lookups, used_joins
+        return value, used_joins
 
     def solve_lookup_type(self, lookup):
         """
@@ -1014,13 +996,11 @@ class Query:
                 return expression_lookups, (), expression
         _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
         field_parts = lookup_splitted[0:len(lookup_splitted) - len(lookup_parts)]
-        if len(lookup_parts) == 0:
-            lookup_parts = ['exact']
-        elif len(lookup_parts) > 1:
-            if not field_parts:
-                raise FieldError(
-                    'Invalid lookup "%s" for model %s".' %
-                    (lookup, self.get_meta().model.__name__))
+        if len(lookup_parts) > 1 and not field_parts:
+            raise FieldError(
+                'Invalid lookup "%s" for model %s".' %
+                (lookup, self.get_meta().model.__name__)
+            )
         return lookup_parts, field_parts, False
 
     def check_query_object_type(self, value, opts, field):
@@ -1063,23 +1043,43 @@ class Query:
         The lookups is a list of names to extract using get_lookup()
         and get_transform().
         """
-        lookups = lookups[:]
-        while lookups:
-            name = lookups[0]
-            # If there is just one part left, try first get_lookup() so
-            # that if the lhs supports both transform and lookup for the
-            # name, then lookup will be picked.
-            if len(lookups) == 1:
-                final_lookup = lhs.get_lookup(name)
-                if not final_lookup:
-                    # We didn't find a lookup. We are going to interpret
-                    # the name as transform, and do an Exact lookup against
-                    # it.
-                    lhs = self.try_transform(lhs, name)
-                    final_lookup = lhs.get_lookup('exact')
-                return final_lookup(lhs, rhs)
+        # __exact is the default lookup if one isn't given.
+        if len(lookups) == 0:
+            lookups = ['exact']
+
+        for name in lookups[:-1]:
             lhs = self.try_transform(lhs, name)
-            lookups = lookups[1:]
+        # First try get_lookup() so that the lookup takes precedence if the lhs
+        # supports both transform and lookup for the name.
+        lookup_class = lhs.get_lookup(lookups[-1])
+        if not lookup_class:
+            if lhs.field.is_relation:
+                raise FieldError('Related Field got invalid lookup: {}'.format(lookups[-1]))
+            # A lookup wasn't found. Try to interpret the name as a transform
+            # and do an Exact lookup against it.
+            lhs = self.try_transform(lhs, lookups[-1])
+            lookup_class = lhs.get_lookup('exact')
+
+        if not lookup_class:
+            return
+
+        lookup = lookup_class(lhs, rhs)
+        # Interpret '__exact=None' as the sql 'is NULL'; otherwise, reject all
+        # uses of None as a query value.
+        if lookup.rhs is None:
+            if lookup.lookup_name not in ('exact', 'iexact'):
+                raise ValueError("Cannot use None as a query value")
+            return lhs.get_lookup('isnull')(lhs, True)
+
+        # For Oracle '' is equivalent to null. The check must be done at this
+        # stage because join promotion can't be done in the compiler. Using
+        # DEFAULT_DB_ALIAS isn't nice but it's the best that can be done here.
+        # A similar thing is done in is_nullable(), too.
+        if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and
+                lookup.lookup_name == 'exact' and lookup.rhs == ''):
+            return lhs.get_lookup('isnull')(lhs, True)
+
+        return lookup
 
     def try_transform(self, lhs, name):
         """
@@ -1133,7 +1133,7 @@ class Query:
 
         # Work out the lookup type and remove it from the end of 'parts',
         # if necessary.
-        value, lookups, used_joins = self.prepare_lookup_value(value, lookups, can_reuse, allow_joins)
+        value, used_joins = self.resolve_lookup_value(value, can_reuse, allow_joins)
 
         clause = self.where_class()
         if reffed_expression:
@@ -1173,25 +1173,19 @@ class Query:
             num_lookups = len(lookups)
             if num_lookups > 1:
                 raise FieldError('Related Field got invalid lookup: {}'.format(lookups[0]))
-            assert num_lookups > 0  # Likely a bug in Django if this fails.
-            lookup_class = field.get_lookup(lookups[0])
-            if lookup_class is None:
-                raise FieldError('Related Field got invalid lookup: {}'.format(lookups[0]))
             if len(targets) == 1:
-                lhs = targets[0].get_col(alias, field)
+                col = targets[0].get_col(alias, field)
             else:
-                lhs = MultiColSource(alias, targets, sources, field)
-            condition = lookup_class(lhs, value)
-            lookup_type = lookup_class.lookup_name
+                col = MultiColSource(alias, targets, sources, field)
         else:
             col = targets[0].get_col(alias, field)
-            condition = self.build_lookup(lookups, col, value)
-            lookup_type = condition.lookup_name
 
+        condition = self.build_lookup(lookups, col, value)
+        lookup_type = condition.lookup_name
         clause.add(condition, AND)
 
-        require_outer = lookup_type == 'isnull' and value is True and not current_negated
-        if current_negated and (lookup_type != 'isnull' or value is False):
+        require_outer = lookup_type == 'isnull' and condition.rhs is True and not current_negated
+        if current_negated and (lookup_type != 'isnull' or condition.rhs is False):
             require_outer = True
             if (lookup_type != 'isnull' and (
                     self.is_nullable(targets[0]) or
diff --git a/tests/lookup/models.py b/tests/lookup/models.py
index 1c7ea799a6..d58d863885 100644
--- a/tests/lookup/models.py
+++ b/tests/lookup/models.py
@@ -44,7 +44,8 @@ class Tag(models.Model):
 
 
 class NulledTextField(models.TextField):
-    pass
+    def get_prep_value(self, value):
+        return None if value == '' else value
 
 
 @NulledTextField.register_lookup
diff --git a/tests/lookup/tests.py b/tests/lookup/tests.py
index 37e98f43e2..7b08c778df 100644
--- a/tests/lookup/tests.py
+++ b/tests/lookup/tests.py
@@ -846,3 +846,13 @@ class LookupTests(TestCase):
         self.assertFalse(Season.objects.filter(nulled_text_field__isnull=True))
         self.assertTrue(Season.objects.filter(nulled_text_field__nulled__isnull=True))
         self.assertTrue(Season.objects.filter(nulled_text_field__nulled__exact=None))
+        self.assertTrue(Season.objects.filter(nulled_text_field__nulled=None))
+
+    def test_custom_field_none_rhs(self):
+        """
+        __exact=value is transformed to __isnull=True if Field.get_prep_value()
+        converts value to None.
+        """
+        season = Season.objects.create(year=2012, nulled_text_field=None)
+        self.assertTrue(Season.objects.filter(pk=season.pk, nulled_text_field__isnull=True))
+        self.assertTrue(Season.objects.filter(pk=season.pk, nulled_text_field=''))