diff --git a/src/common-principles.md b/src/common-principles.md
index 69aa738149..2565b8076c 100644
--- a/src/common-principles.md
+++ b/src/common-principles.md
@@ -709,7 +709,7 @@ for naming of participants, sessions, acquisition schemes.
 Note that they MUST consist only of allowed characters as described in
 [Definitions](common-principles.md#definitions) above.
 In `<index>`es we RECOMMEND using zero padding (for example, `01` instead of `1`
-if you have more than ten subjects) to make alphabetical sorting more intuitive.
+if some participants have two-digit labels) to make alphabetical sorting more intuitive.
 Note that zero padding SHOULD NOT be used to merely maintain uniqueness
 of `<index>`es.
 
diff --git a/src/schema/objects/formats.yaml b/src/schema/objects/formats.yaml
index 71e613a871..e0704bee31 100644
--- a/src/schema/objects/formats.yaml
+++ b/src/schema/objects/formats.yaml
@@ -5,7 +5,7 @@ index:
   display_name: Index
   description: |
     Non-negative, optionally prefixed with leading zeros for better visual homogeneity and sorting.
-  pattern: '[0-9]*'
+  pattern: '[0-9]+'
 label:
   display_name: Label
   description: |