diff --git a/src/icalendar/cal.py b/src/icalendar/cal.py
index 9bfed80..3b05866 100644
--- a/src/icalendar/cal.py
+++ b/src/icalendar/cal.py
@@ -358,6 +358,10 @@ class Component(CaselessDict):
             elif uname == 'END':
                 # we are done adding properties to this component
                 # so pop it from the stack and add it to the new top.
+                if not stack:
+                    # The stack is currently empty, the input must be invalid
+                    raise ValueError('END encountered without an accompanying BEGIN!')
+
                 component = stack.pop()
                 if not stack:  # we are at the end
                     comps.append(component)
diff --git a/src/icalendar/fuzzing/ical_fuzzer.py b/src/icalendar/fuzzing/ical_fuzzer.py
index d524920..bffd078 100644
--- a/src/icalendar/fuzzing/ical_fuzzer.py
+++ b/src/icalendar/fuzzing/ical_fuzzer.py
@@ -17,16 +17,24 @@
 import atheris
 import sys
 
-with atheris.instrument_imports(include=['icalendar']):
-    from icalendar import Calendar
+with atheris.instrument_imports(
+        include=['icalendar'],
+        exclude=['pytz', 'six', 'site_packages', 'pkg_resources', 'dateutil']):
+    import icalendar
+
+_value_error_matches = [
+    "component", "parse", "Expected", "Wrong date format", "END encountered",
+    "vDDD", 'recurrence', 'Wrong datetime', 'Offset must', 'Invalid iCalendar'
+]
 
 
+@atheris.instrument_func
 def TestOneInput(data):
     fdp = atheris.FuzzedDataProvider(data)
     try:
         b = fdp.ConsumeBool()
 
-        cal = Calendar.from_ical(fdp.ConsumeString(fdp.remaining_bytes()))
+        cal = icalendar.Calendar.from_ical(fdp.ConsumeString(fdp.remaining_bytes()))
 
         if b:
             for event in cal.walk('VEVENT'):
@@ -34,10 +42,11 @@ def TestOneInput(data):
         else:
             cal.to_ical()
     except ValueError as e:
-        if "component" in str(e) or "parse" in str(e) or "Expected" in str(e):
+        if any(m in str(e) for m in _value_error_matches):
             return -1
         raise e
 
+
 def main():
     atheris.Setup(sys.argv, TestOneInput)
     atheris.Fuzz()
diff --git a/src/icalendar/prop.py b/src/icalendar/prop.py
index c7e7840..f078d74 100644
--- a/src/icalendar/prop.py
+++ b/src/icalendar/prop.py
@@ -250,8 +250,19 @@ class vDDDLists:
         self.dts = vDDD
 
     def to_ical(self):
-        dts_ical = (dt.to_ical() for dt in self.dts)
-        return b",".join(dts_ical)
+        dts_ical = [dt.to_ical() for dt in self.dts]
+
+        # Make sure all elements are of the same type
+        if dts_ical and all(type(dt) is type(dts_ical[0]) for dt in dts_ical):
+            first_dt = dts_ical[0]
+            if isinstance(first_dt, bytes):
+                return b",".join(dts_ical)
+            elif isinstance(first_dt, str):
+                return ",".join(dts_ical)
+            else:
+                raise ValueError(f"Unexpected type {type(first_dt)} in vDDD list!")
+        else:
+            raise ValueError("Type mismatch in vDDD list!")
 
     @staticmethod
     def from_ical(ical, timezone=None):