diff --git a/src/drf_yasg/inspectors/field.py b/src/drf_yasg/inspectors/field.py
index 6e457c2..13913ed 100644
--- a/src/drf_yasg/inspectors/field.py
+++ b/src/drf_yasg/inspectors/field.py
@@ -459,7 +459,7 @@ def get_origin_type(hint_class):
     return getattr(hint_class, '__origin__', None) or hint_class
 
 
-def is_origin_type_subclasses(hint_class, check_class):
+def hint_class_issubclass(hint_class, check_class):
     origin_type = get_origin_type(hint_class)
     return inspect.isclass(origin_type) and issubclass(origin_type, check_class)
 
@@ -480,13 +480,13 @@ if typing:
     def inspect_collection_hint_class(hint_class):
         args = hint_class.__args__
         child_class = args[0] if args else str
-        child_type_info = get_basic_type_info_from_hint(child_class)
-        if not child_type_info:
-            child_type_info = {'type': openapi.TYPE_STRING}
+        child_type_info = get_basic_type_info_from_hint(child_class) or {'type': openapi.TYPE_STRING}
+
         return OrderedDict([
             ('type', openapi.TYPE_ARRAY),
             ('items', openapi.Items(**child_type_info)),
         ])
+
     hinting_type_info.append(((typing.Sequence, typing.AbstractSet), inspect_collection_hint_class))
 
 
@@ -500,18 +500,24 @@ def get_basic_type_info_from_hint(hint_class):
     :rtype: OrderedDict
     """
     if typing and get_origin_type(hint_class) == typing.Union:
-        if len(hint_class.__args__) == 2 and hint_class.__args__[1] == type(None):
+        # Optional is implemented as Union[T, None]
+        if len(hint_class.__args__) == 2 and hint_class.__args__[1] == type(None):  # noqa: E721
             child_type = hint_class.__args__[0]
-            return get_basic_type_info_from_hint(child_type)
+            result = get_basic_type_info_from_hint(child_type)
+            result['x-nullable'] = True
+            return result
+
         return None
 
     for check_class, info in hinting_type_info:
-        if is_origin_type_subclasses(hint_class, check_class):
+        if hint_class_issubclass(hint_class, check_class):
             if callable(info):
                 return info(hint_class)
+
             swagger_type, format = info
             if callable(swagger_type):
                 swagger_type = swagger_type()
+
             return OrderedDict([
                 ('type', swagger_type),
                 ('format', format),
diff --git a/tests/test_get_basic_type_info_from_hint.py b/tests/test_get_basic_type_info_from_hint.py
index 49c1f96..db45fef 100644
--- a/tests/test_get_basic_type_info_from_hint.py
+++ b/tests/test_get_basic_type_info_from_hint.py
@@ -11,7 +11,6 @@ try:
 except ImportError:
     typing = None
 
-
 if typing:
     @pytest.mark.parametrize('hint_class, expected_swagger_type_info', [
         (int, {'type': openapi.TYPE_INTEGER, 'format': None}),
@@ -24,9 +23,13 @@ if typing:
         (List[str], {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_STRING)}),
         (List[bool], {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_BOOLEAN)}),
         (Set[int], {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_INTEGER)}),
-        (Optional[bool], {'type': openapi.TYPE_BOOLEAN, 'format': None}),
-        (Optional[List[int]], {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_INTEGER)}),
-        (Union[List[int], type(None)], {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_INTEGER)}),
+        (Optional[bool], {'type': openapi.TYPE_BOOLEAN, 'format': None, 'x-nullable': True}),
+        (Optional[List[int]], {
+            'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_INTEGER), 'x-nullable': True
+        }),
+        (Union[List[int], type(None)], {
+            'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_INTEGER), 'x-nullable': True
+        }),
         # Following cases are not 100% correct, but it should work somehow and not crash.
         (Union[int, float], None),
         (List, {'type': openapi.TYPE_ARRAY, 'items': openapi.Items(openapi.TYPE_STRING)}),