diff --git a/harness/communication.org b/harness/communication.org
index 7159006..e69d347 100644
--- a/harness/communication.org
+++ b/harness/communication.org
@@ -84,7 +84,7 @@ The ~communication.lisp~ module defines the low-level transport and framing logi
 
 (defun validate-communication-protocol-schema (msg)
   "Strict structural validation for incoming protocol messages."
-  (unless (listp msg) (error "Message must be a plist)
+  (unless (listp msg) (error "Message must be a plist"))
   (let ((type (proto-get msg :type)))
     (unless (member type '(:REQUEST :EVENT :RESPONSE :LOG :STATUS))
       (error "Invalid message type '~a'" type))