diff --git a/vdirsyncer/cli/fetchparams.py b/vdirsyncer/cli/fetchparams.py
index ebdceff..cbd76b8 100644
--- a/vdirsyncer/cli/fetchparams.py
+++ b/vdirsyncer/cli/fetchparams.py
@@ -4,7 +4,7 @@ import click
 
 from . import AppContext
 from .. import exceptions, log
-from ..utils import expand_path
+from ..utils import expand_path, synchronized
 
 SUFFIX = '.fetch'
 
@@ -31,6 +31,7 @@ def expand_fetch_params(config):
     return config
 
 
+@synchronized()
 def _fetch_value(opts, key):
     if not isinstance(opts, list):
         raise ValueError('Invalid value for {}: Expected a list, found {!r}.'
diff --git a/vdirsyncer/utils/__init__.py b/vdirsyncer/utils/__init__.py
index bd2eace..37e2d13 100644
--- a/vdirsyncer/utils/__init__.py
+++ b/vdirsyncer/utils/__init__.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 
+import functools
 import os
 import sys
 import uuid
@@ -178,3 +179,17 @@ def generate_href(ident=None, safe=SAFE_UID_CHARS):
         return to_unicode(uuid.uuid4().hex)
     else:
         return ident
+
+
+def synchronized(lock=None):
+    if lock is None:
+        from threading import Lock
+        lock = Lock()
+
+    def inner(f):
+        @functools.wraps(f)
+        def wrapper(*args, **kwargs):
+            with lock:
+                return f(*args, **kwargs)
+        return wrapper
+    return inner