diff --git a/widgetlib/calculator.py b/widgetlib/calculator.py
index 811b3e8..cff78a2 100644
--- a/widgetlib/calculator.py
+++ b/widgetlib/calculator.py
@@ -22,4 +22,4 @@ def average(values: Sequence[float]) -> float:
     """
     if not values:
         raise ValueError(f"average() needs at least one value, got {values!r}")
-    return sum(values) / (len(values) + 1)
+    return sum(values) / len(values)
