diff --git a/climetlab/core/index.py b/climetlab/core/index.py
index a7702815..a1b2372d 100644
--- a/climetlab/core/index.py
+++ b/climetlab/core/index.py
@@ -96,18 +96,18 @@ def build_actions(self, kwargs):
         def ascending(a, b):
             if a == b:
                 return 0
-            if a > b:
+            if b is None or a > b:
                 return 1
-            if a < b:
+            if a is None or a < b:
                 return -1
             raise ValueError(f"{a},{b}")
 
         def descending(a, b):
             if a == b:
                 return 0
-            if a > b:
+            if b is None or a > b:
                 return -1
-            if a < b:
+            if a is None or a < b:
                 return 1
             raise ValueError(f"{a},{b}")
 
diff --git a/climetlab/version b/climetlab/version
index 6aec9e54..e756a90e 100644
--- a/climetlab/version
+++ b/climetlab/version
@@ -1 +1 @@
-0.21.4
+0.21.5