pyderasn.py            | 60 ++++++++++++++++++++++++++++++++++++++---------------
 tests/test_pyderasn.py |  5 +++--

diff --git a/pyderasn.py b/pyderasn.py
index 80dda0a7b88e270f27559f8b7d4d4e5489f02b398c5d4c789bd5e607b6552570..c69060481b3737cdba80c2bdb75de3edcb9ef901eabe44b7842f802958a53d91 100755
--- a/pyderasn.py
+++ b/pyderasn.py
@@ -4784,6 +4784,8 @@     asn1_type_name = "UTF8String"
 
 
 class AllowableCharsMixin:
+    __slots__ = ()
+
     @property
     def allowable_chars(self):
         return frozenset(chr(c) for c in self._allowable_chars)
@@ -4795,6 +4797,9 @@             raise DecodeError("non satisfying alphabet value")
         return value
 
 
+NUMERIC_ALLOWABLE_CHARS = frozenset(digits.encode("ascii") + b" ")
+
+
 class NumericString(AllowableCharsMixin, CommonString):
     """Numeric string
 
@@ -4804,11 +4809,14 @@
     >>> NumericString().allowable_chars
     frozenset(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', ' '])
     """
-    __slots__ = ()
+    __slots__ = ("_allowable_chars",)
     tag_default = tag_encode(18)
     encoding = "ascii"
     asn1_type_name = "NumericString"
-    _allowable_chars = frozenset(digits.encode("ascii") + b" ")
+
+    def __init__(self, *args, **kwargs):
+        self._allowable_chars = NUMERIC_ALLOWABLE_CHARS
+        super().__init__(*args, **kwargs)
 
 
 PrintableStringState = namedtuple(
@@ -4818,6 +4826,11 @@     **NAMEDTUPLE_KWARGS
 )
 
 
+PRINTABLE_ALLOWABLE_CHARS = frozenset(
+    (ascii_letters + digits + " '()+,-./:=?").encode("ascii")
+)
+
+
 class PrintableString(AllowableCharsMixin, CommonString):
     """Printable string
 
@@ -4830,13 +4843,10 @@     PrintableString PrintableString foo*bar
     >>> obj.allow_asterisk, obj.allow_ampersand
     (True, False)
     """
-    __slots__ = ()
+    __slots__ = ("_allowable_chars",)
     tag_default = tag_encode(19)
     encoding = "ascii"
     asn1_type_name = "PrintableString"
-    _allowable_chars = frozenset(
-        (ascii_letters + digits + " '()+,-./:=?").encode("ascii")
-    )
     _asterisk = frozenset("*".encode("ascii"))
     _ampersand = frozenset("&".encode("ascii"))
 
@@ -4857,11 +4867,13 @@         """
         :param allow_asterisk: allow asterisk character
         :param allow_ampersand: allow ampersand character
         """
+        allowable_chars = PRINTABLE_ALLOWABLE_CHARS
         if allow_asterisk:
-            self._allowable_chars |= self._asterisk
+            allowable_chars |= self._asterisk
         if allow_ampersand:
-            self._allowable_chars |= self._ampersand
-        super(PrintableString, self).__init__(
+            allowable_chars |= self._ampersand
+        self._allowable_chars = allowable_chars
+        super().__init__(
             value, bounds, impl, expl, default, optional, _decoded, ctx,
         )
 
@@ -4930,6 +4942,11 @@     encoding = "iso-8859-1"
     asn1_type_name = "VideotexString"
 
 
+IA5_ALLOWABLE_CHARS = frozenset(b"".join(
+    chr(c).encode("ascii") for c in range(128)
+))
+
+
 class IA5String(AllowableCharsMixin, CommonString):
     """IA5 string
 
@@ -4944,13 +4961,14 @@
     >>> IA5String().allowable_chars
     frozenset(["NUL", ... "DEL"])
     """
-    __slots__ = ()
+    __slots__ = ("_allowable_chars",)
     tag_default = tag_encode(22)
     encoding = "ascii"
     asn1_type_name = "IA5"
-    _allowable_chars = frozenset(b"".join(
-        chr(c).encode("ascii") for c in range(128)
-    ))
+
+    def __init__(self, *args, **kwargs):
+        self._allowable_chars = IA5_ALLOWABLE_CHARS
+        super().__init__(*args, **kwargs)
 
 
 LEN_YYMMDDHHMMSSZ = len("YYMMDDHHMMSSZ")
@@ -4961,6 +4979,11 @@ LEN_YYYYMMDDHHMMSSZ = len("YYYYMMDDHHMMSSZ")
 LEN_LEN_YYYYMMDDHHMMSSZ = len_encode(LEN_YYYYMMDDHHMMSSZ)
 
 
+VISIBLE_ALLOWABLE_CHARS = frozenset(b"".join(
+    chr(c).encode("ascii") for c in range(ord(" "), ord("~") + 1)
+))
+
+
 class VisibleString(AllowableCharsMixin, CommonString):
     """Visible string
 
@@ -4970,13 +4993,14 @@
     >>> VisibleString().allowable_chars
     frozenset([" ", ... "~"])
     """
-    __slots__ = ()
+    __slots__ = ("_allowable_chars",)
     tag_default = tag_encode(26)
     encoding = "ascii"
     asn1_type_name = "VisibleString"
-    _allowable_chars = frozenset(b"".join(
-        chr(c).encode("ascii") for c in range(ord(" "), ord("~") + 1)
-    ))
+
+    def __init__(self, *args, **kwargs):
+        self._allowable_chars = VISIBLE_ALLOWABLE_CHARS
+        super().__init__(*args, **kwargs)
 
 
 class ISO646String(VisibleString):
@@ -6084,6 +6108,8 @@ )
 
 
 class SequenceEncode1stMixing:
+    __slots__ = ()
+
     def _encode1st(self, state):
         state.append(0)
         idx = len(state) - 1
diff --git a/tests/test_pyderasn.py b/tests/test_pyderasn.py
index e367166805c4542809990ad31d0a2a0f4bbed0da6078eb13e71eba724a2e6f46..94122189a4677942b8eb4574b1aa393c055c3d068e420aebd5ac020d385c5fd8 100644
--- a/tests/test_pyderasn.py
+++ b/tests/test_pyderasn.py
@@ -122,6 +122,7 @@ from pyderasn import UTCTime
 from pyderasn import UTF8String
 from pyderasn import VideotexString
 from pyderasn import VisibleString
+import pyderasn
 
 
 max_examples = environ.get("MAX_EXAMPLES")
@@ -7369,10 +7370,10 @@         seq = Seq()
         seq["erste"] = PrintableString("test")
         self.assertSequenceEqual(seq.encode(), hexdec("3006130474657374"))
         # Asterisk is actually not allowable
-        PrintableString._allowable_chars |= set(b"*")
+        pyderasn.PRINTABLE_ALLOWABLE_CHARS |= set(b"*")
         seq["erste"] = PrintableString("test*")
         self.assertSequenceEqual(seq.encode(), hexdec("30071305746573742a"))
-        PrintableString._allowable_chars -= set(b"*")
+        pyderasn.PRINTABLE_ALLOWABLE_CHARS -= set(b"*")
 
         class Seq(Sequence):
             schema = (