diff --git a/tests.py b/tests.py
index 8b6deb4..0c1a6e6 100644
--- a/tests.py
+++ b/tests.py
@@ -864,6 +864,34 @@ def test_issue50(self):
         cal = base.readOne(test_file)
         self.assertEqual(datetime.datetime(2024, 8, 12, 22, 30, tzinfo=tzutc()), cal.vevent.dtend.value)
 
+    def test_nested_begin_depth(self):
+        """
+        Catch deeply nested BEGIN and raise ParseError rather than
+        exceeding interpreter recursion limit.
+
+        Reported by: oc-8a8a9d
+        """
+        depth = 200
+        deep_ical = ("".join("BEGIN:X\r\n" for _ in range(depth))
+            + "FN:test\r\n"
+            + "".join("END:X\r\n" for _ in range(depth)))
+        obj = None
+
+        default_recursion_limit = sys.getrecursionlimit()
+        sys.setrecursionlimit(150)
+        try:
+            # Vector 1: default readOne (transform=True)
+            with self.assertRaises(ParseError):
+                obj = base.readOne(deep_ical)
+
+            # Vector 2: serialize()
+            if obj:
+                with self.assertRaises(ParseError):
+                    obj.serialize()
+
+        finally:
+            sys.setrecursionlimit(default_recursion_limit)
+
 
 class TestChangeTZ(unittest.TestCase):
     """
diff --git a/vobject/base.py b/vobject/base.py
index 6765608..77010ac 100644
--- a/vobject/base.py
+++ b/vobject/base.py
@@ -87,6 +87,15 @@ def to_basestring(s):
 TAB = '\t'
 SPACEORTAB = SPACE + TAB
 
+# Maximum allowed nesting depth of components (nested BEGIN:/END:
+# blocks).  Deeply-nested input would otherwise build a component tree
+# that overflows the interpreter stack during the recursive
+# transformChildrenToNative()/serialize() walks, raising an uncaught
+# RecursionError (a denial of service). Instead we raise the
+# documented ParseError once this depth is exceeded. The limit is far
+# above any legitimate vCard/iCal nesting.
+DEFAULT_MAX_NESTING = 100
+
 # --------------------------------- Main classes -------------------------------
 
 
@@ -1077,7 +1086,8 @@ def pop(self):
 
 
 def readComponents(streamOrString, validate=False, transform=True,
-                   ignoreUnreadable=False, allowQP=False):
+                   ignoreUnreadable=False, allowQP=False,
+                   max_nesting=DEFAULT_MAX_NESTING):
     """
     Generate one Component at a time from a stream.
     """
@@ -1107,6 +1117,9 @@ def readComponents(streamOrString, validate=False, transform=True,
                 versionLine = vline
                 stack.modifyTop(vline)
             elif vline.name == "BEGIN":
+                if len(stack) >= max_nesting:
+                    raise ParseError("Component nesting depth exceeds requested "
+                                     "maximum of {0} levels".format(max_nesting), n)
                 stack.push(Component(vline.value, group=vline.group))
             elif vline.name == "PROFILE":
                 if not stack.top():
diff --git a/vobject/icalendar.py b/vobject/icalendar.py
index 325bae7..52ff94b 100644
--- a/vobject/icalendar.py
+++ b/vobject/icalendar.py
@@ -80,7 +80,7 @@ def getTzid(tzid, smart=True):
                 tz = timezone(tzid)
                 registerTzid(toUnicode(tzid), tz)
             except UnknownTimeZoneError as e:
-                logging.error(e)
+                logging.error("Unknown Timezone: %r", e.args[0])
         except ImportError as e:
             logging.error(e)
     return tz
@@ -335,6 +335,10 @@ def pickTzid(tzinfo, allowUTC=False):
         if tzinfo is None or (not allowUTC and tzinfo_eq(tzinfo, utc)):
             return None
 
+        # Try a zoneinfo (CPython 3.9+) first.
+        if hasattr(tzinfo, 'key'):
+            return toUnicode(tzinfo.key)
+
         # Try pytz tzid key
         if hasattr(tzinfo, 'tzid'):
             return toUnicode(tzinfo.tzid)