diff --git a/proto/protocol.xml b/proto/protocol.xml
index 64a481d..9deb324 100644
--- a/proto/protocol.xml
+++ b/proto/protocol.xml
@@ -238,9 +238,7 @@
         EIS implementation after the callback is fired and as such the client must not
         attempt to use it after that point.
 
-        <!-- FIXME: this needs to be something useful, but we don't have anything
-             useful to send yet -->
-        The callback_data passed in the callback is always zero.
+        The callback_data in the ei_callback.done event is always zero.
 
         Note that for a client to use this request it must announce
         support for the "ei_callback" interface in ei_handshake.interface_version.
@@ -366,6 +364,9 @@
         ei client implementation after the callback is fired and as
         such the client must not attempt to use it after that point.
 
+        The callback_data in the resulting ei_pingpong.done request is
+        ignored by the EIS implementation.
+
         Note that for a EIS implementation to use this request the client must
         announce support for this interface in ei_handshake.interface_version. It is
         a protocol violation to send this event to a client without the