```diff
--- stats.py
+++ stats.py
@@ -1,4 +1,9 @@
 def median(values):
+    if not values:
+        return None
     values = sorted(values)
-    mid = len(values) // 2
-    return values[mid]
+    n = len(values)
+    mid = n // 2
+    if n % 2 == 0:
+        return (values[mid - 1] + values[mid]) / 2
+    return values[mid]

```