diff --git a/secop/datatypes.py b/secop/datatypes.py
index 3f76544..885ccd2 100644
--- a/secop/datatypes.py
+++ b/secop/datatypes.py
@@ -260,7 +260,7 @@ class StringType(DataType):
     minsize = None
     maxsize = None
 
-    def __init__(self, maxsize=None, minsize=0):
+    def __init__(self, maxsize=255, minsize=0):
         if maxsize is None:
             raise ValueError('StringType needs a maximum bytes count!')
         minsize, maxsize = min(minsize, maxsize), max(minsize, maxsize)
diff --git a/test/test_datatypes.py b/test/test_datatypes.py
index 8b39adc..b8c9143 100644
--- a/test/test_datatypes.py
+++ b/test/test_datatypes.py
@@ -145,8 +145,7 @@ def test_BLOBType():
 
 def test_StringType():
     # test constructor catching illegal arguments
-    with pytest.raises(ValueError):
-        dt = StringType()
+    dt = StringType()
     dt = StringType(12)
     assert dt.as_json == ['string', 12]