diff --git a/pkg/pmapi/messages.go b/pkg/pmapi/messages.go
index 892b55c0..b97d3e02 100644
--- a/pkg/pmapi/messages.go
+++ b/pkg/pmapi/messages.go
@@ -147,7 +147,7 @@ const ConversationIDDomain = `protonmail.conversationid`
 // InternalIDDomain is used as a placeholder for reference/message ID headers to improve compatibility with various clients.
 const InternalIDDomain = `protonmail.internalid`
 
-// InternalReferenceFormat describes format of the message ID (as regex) used for parsing reference headers
+// InternalReferenceFormat describes format of the message ID (as regex) used for parsing reference headers.
 const InternalReferenceFormat = `(?U)<.*@` + InternalIDDomain + `>`
 
 // Message structure.