diff --git a/sqlalchemy_utils/types/timezone.py b/sqlalchemy_utils/types/timezone.py
index 239712c..b474066 100644
--- a/sqlalchemy_utils/types/timezone.py
+++ b/sqlalchemy_utils/types/timezone.py
@@ -52,9 +52,10 @@ class TimezoneType(types.TypeDecorator, ScalarCoercible):
 
         elif backend == 'pytz':
             try:
-                from pytz import tzfile, timezone
+                from pytz import timezone
+                from pytz.tzinfo import BaseTzInfo
 
-                self.python_type = tzfile.DstTzInfo
+                self.python_type = BaseTzInfo
                 self._to = timezone
                 self._from = six.text_type
 
@@ -71,13 +72,11 @@ class TimezoneType(types.TypeDecorator, ScalarCoercible):
             )
 
     def _coerce(self, value):
-        if value and not isinstance(value, self.python_type):
+        if value is not None and not isinstance(value, self.python_type):
             obj = self._to(value)
             if obj is None:
                 raise ValueError("unknown time zone '%s'" % value)
-
             return obj
-
         return value
 
     def process_bind_param(self, value, dialect):
diff --git a/tests/types/test_timezone.py b/tests/types/test_timezone.py
index 502288f..6124688 100644
--- a/tests/types/test_timezone.py
+++ b/tests/types/test_timezone.py
@@ -1,7 +1,9 @@
 import pytest
+import pytz
 import sqlalchemy as sa
+from dateutil.zoneinfo import getzoneinfofile_stream, tzfile, ZoneInfoFile
 
-from sqlalchemy_utils.types import timezone
+from sqlalchemy_utils.types import timezone, TimezoneType
 
 
 @pytest.fixture
@@ -46,3 +48,48 @@ class TestTimezoneType(object):
 
         assert visitor_dateutil is not None
         assert visitor_pytz is not None
+
+
+TIMEZONE_BACKENDS = ['dateutil', 'pytz']
+
+
+def test_can_coerce_pytz_DstTzInfo():
+    tzcol = TimezoneType(backend='pytz')
+    tz = pytz.timezone('America/New_York')
+    assert isinstance(tz, pytz.tzfile.DstTzInfo)
+    assert tzcol._coerce(tz) is tz
+
+
+def test_can_coerce_pytz_StaticTzInfo():
+    tzcol = TimezoneType(backend='pytz')
+    tz = pytz.timezone('Pacific/Truk')
+    assert isinstance(tz, pytz.tzfile.StaticTzInfo)
+    assert tzcol._coerce(tz) is tz
+
+
+@pytest.mark.parametrize('zone', pytz.all_timezones)
+def test_can_coerce_string_for_pytz_zone(zone):
+    tzcol = TimezoneType(backend='pytz')
+    assert tzcol._coerce(zone).zone == zone
+
+
+@pytest.mark.parametrize(
+    'zone', ZoneInfoFile(getzoneinfofile_stream()).zones.keys())
+def test_can_coerce_string_for_dateutil_zone(zone):
+    tzcol = TimezoneType(backend='dateutil')
+    assert isinstance(tzcol._coerce(zone), tzfile)
+
+
+@pytest.mark.parametrize('backend', TIMEZONE_BACKENDS)
+def test_can_coerce_and_raise_UnknownTimeZoneError_or_ValueError(backend):
+    tzcol = TimezoneType(backend=backend)
+    with pytest.raises((ValueError, pytz.exceptions.UnknownTimeZoneError)):
+        tzcol._coerce('SolarSystem/Mars')
+    with pytest.raises((ValueError, pytz.exceptions.UnknownTimeZoneError)):
+        tzcol._coerce('')
+
+
+@pytest.mark.parametrize('backend', TIMEZONE_BACKENDS)
+def test_can_coerce_None(backend):
+    tzcol = TimezoneType(backend=backend)
+    assert tzcol._coerce(None) is None