diff --git a/radicale/auth/IMAP.py b/radicale/auth/IMAP.py
index 24c0d06..a866f30 100644
--- a/radicale/auth/IMAP.py
+++ b/radicale/auth/IMAP.py
@@ -43,6 +43,8 @@ IMAP_USE_SSL = config.get("auth", "imap_ssl")
 
 def is_authenticated(user, password):
     """Check if ``user``/``password`` couple is valid."""
+    if not user or not password:
+        return False
 
     log.LOGGER.debug(
         "Connecting to IMAP server %s:%s." % (IMAP_SERVER, IMAP_SERVER_PORT,))
diff --git a/radicale/auth/courier.py b/radicale/auth/courier.py
index 3be5811..ef833b7 100644
--- a/radicale/auth/courier.py
+++ b/radicale/auth/courier.py
@@ -32,6 +32,8 @@ COURIER_SOCKET = config.get("auth", "courier_socket")
 
 def is_authenticated(user, password):
     """Check if ``user``/``password`` couple is valid."""
+    if not user or not password:
+        return False
 
     line = "%s\nlogin\n%s\n%s" % (sys.argv[0], user, password)
     line = "AUTH %i\n%s" % (len(line), line)