diff --git a/specification/common/README.md b/specification/common/README.md
index 1b46da3e628..1ac49745738 100644
--- a/specification/common/README.md
+++ b/specification/common/README.md
@@ -28,6 +28,7 @@ aliases: [/docs/reference/specification/common/common]
 An `Attribute` is a key-value pair, which MUST have the following properties:
 
 - The attribute key MUST be a non-`null` and non-empty string.
+  - Attribute keys are case-sensitive.
 - The attribute value is either:
   - A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
   - An array of primitive type values. The array MUST be homogeneous,