diff --git a/dataset/types.py b/dataset/types.py
index 904ffa9..d30d3c1 100644
--- a/dataset/types.py
+++ b/dataset/types.py
@@ -2,6 +2,7 @@ from datetime import datetime, date
 
 from sqlalchemy import Integer, UnicodeText, Float, BigInteger
 from sqlalchemy import Boolean, Date, DateTime, Unicode, JSON
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.types import TypeEngine
 
 
@@ -16,25 +17,21 @@ class Types(object):
     date = Date
     datetime = DateTime
 
-    def __init__(self, dialect = None):
+    def __init__(self, dialect=None):
         self._dialect = dialect
 
     @property
     def json(self):
         if self._dialect is not None and self._dialect == 'postgresql':
-            from sqlalchemy.dialects.postgresql import JSONB
             return JSONB
         return JSON
 
-    def guess(self, sample, dialect = None):
+    def guess(self, sample):
         """Given a single sample, guess the column type for the field.
 
         If the sample is an instance of an SQLAlchemy type, the type will be
         used instead.
         """
-        if dialect is not None:
-            self._dialect = dialect
-
         if isinstance(sample, TypeEngine):
             return sample
         if isinstance(sample, bool):