diff --git a/sqlalchemy_pytds/dialect.py b/sqlalchemy_pytds/dialect.py
index def555e..b54f253 100644
--- a/sqlalchemy_pytds/dialect.py
+++ b/sqlalchemy_pytds/dialect.py
@@ -171,10 +171,10 @@ def do_execute(self, cursor, statement, parameters, context=None):
                         v = parameters.get(name, None)
                         #print('cvt', name, v)
                         if isinstance(v, str):
-                            assert len(v) < 8000
+                            assert len(v) <= 4000
                             parameters[name] = tds_base.Param(name=name, type=tds_types.NVarCharType(size=len(v)), value=v, flags=0)
                         elif isinstance(v, bytes):
-                            assert len(v) < 8000
+                            assert len(v) <= 8000
                             parameters[name] = tds_base.Param(name=name, type=tds_types.VarBinaryType(size=len(v)), value=v, flags=0)
             #print('parm:', parameters)
         cursor.execute(statement, parameters)