diff --git a/icalendar_timeseries_server/cal.py b/icalendar_timeseries_server/cal.py
index fb5269d..0e582ec 100644
--- a/icalendar_timeseries_server/cal.py
+++ b/icalendar_timeseries_server/cal.py
@@ -3,7 +3,7 @@ from typing import Dict, List, Iterable
 import sys
 import urllib.request
 from datetime import datetime, date, timedelta
-from threading import Timer
+from threading import Lock, Timer
 
 from dateutil import rrule
 from icalendar import cal
@@ -15,6 +15,7 @@ from icalendar_timeseries_server.event import Event
 
 
 _SCRAPE_CACHE: Dict[str, List[Event]] = dict()
+_SCRAPE_CACHE_LOCK: Lock = Lock()
 
 __py_version: str = f'{sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}'
 USER_AGENT: str = f'icalendar-timeseries-server/{__version__} (Python/{__py_version})'
@@ -49,7 +50,7 @@ def _parse_recurring(event: cal.Event, start: datetime, end: datetime, duration:
 
 
 def _scrape_calendar(name: str, config: CalendarConfig, start: datetime, end: datetime):
-    global _SCRAPE_CACHE
+    global _SCRAPE_CACHE, _SCRAPE_CACHE_LOCK
     events = []
 
     opener: urllib.request.OpenerDirector = config.get_url_opener()
@@ -79,7 +80,8 @@ def _scrape_calendar(name: str, config: CalendarConfig, start: datetime, end: da
             for occurence in occurences:
                 if start <= occurence < end:
                     events.append(Event(name, element, occurence, occurence + duration))
-    _SCRAPE_CACHE[name] = events
+    with _SCRAPE_CACHE_LOCK:
+        _SCRAPE_CACHE[name] = events
 
 
 def scrape_calendar(name: str, config: CalendarConfig):
@@ -110,4 +112,5 @@ def start_scrape_calendar(name: str, config: CalendarConfig):
 
 def get_calendar(name: str):
     global _SCRAPE_CACHE
-    return _SCRAPE_CACHE.get(name, [])
+    with _SCRAPE_CACHE_LOCK:
+        return _SCRAPE_CACHE.get(name, [])