diff --git a/Mailbox-Properties-Client.md b/Mailbox-Properties-Client.md
index 5dbb5b5..fb6567f 100644
--- a/Mailbox-Properties-Client.md
+++ b/Mailbox-Properties-Client.md
@@ -24,7 +24,7 @@ For major version 0, the `properties` dictionary of a well-formed update contain
 
 `clientSupports` is a non-empty list of Mailbox API versions that the sender can talk to as a client. Each element is a list with 2 elements: `maj` (int), `min` (int).
 
-`serverSupports` is a list of Mailbox API versions that the sender's mailbox supports. The list must be empty if `properties` is empty (there is no mailbox). Each element is a list with 2 elements: `maj` (int), `min` (int).
+`serverSupports` is a list of Mailbox API versions that the sender's mailbox supports. The list can only be empty if `properties` is empty (there is no mailbox). Each element is a list with 2 elements: `maj` (int), `min` (int).
 
 ### Validity policy