diff --git a/src/akkudoktoreosserver/flask_server.py b/src/akkudoktoreosserver/flask_server.py
index 90111778..c97373fd 100755
--- a/src/akkudoktoreosserver/flask_server.py
+++ b/src/akkudoktoreosserver/flask_server.py
@@ -2,7 +2,7 @@
 
 import os
 from datetime import datetime
-from typing import Any
+from typing import Any, TypeGuard
 
 import matplotlib
 
@@ -27,7 +27,7 @@
 )
 
 
-def isfloat(num: Any) -> bool:
+def isfloat(num: Any) -> TypeGuard[float]:
     """Check if a given input can be converted to float."""
     if num is None:
         return False