diff --git a/fritz_temp_sync/fritzbox.py b/fritz_temp_sync/fritzbox.py
index 49262e9..36e1191 100755
--- a/fritz_temp_sync/fritzbox.py
+++ b/fritz_temp_sync/fritzbox.py
@@ -42,7 +42,7 @@ class FritzBox:
 
         logging.debug(f"Calculate v2 challenge: {challenge}")
         chall_regex = re.compile(
-            "2\$(?P<iter1>[0-9a-zA-Z]+)\$(?P<salt1>[0-9a-zA-Z]+)\$(?P<iter2>[0-9a-zA-Z]+)\$(?P<salt2>[0-9a-zA-Z]+)")
+            r"2\$(?P<iter1>[0-9a-zA-Z]+)\$(?P<salt1>[0-9a-zA-Z]+)\$(?P<iter2>[0-9a-zA-Z]+)\$(?P<salt2>[0-9a-zA-Z]+)")
 
         chall_parts = chall_regex.match(challenge).groupdict()
         salt1: bytes = bytes.fromhex(chall_parts["salt1"])