new revision of content node fails to update node_revisions.type

i have a custom content module. when i create a node of this type, the node.type
field is correctly initialized. but when i create a revision of that same node,
the node_revisions.type field remains empty.

since a user cannot modify the content type from the gui, i don't understand why
the 'type' field even exists in node_revisions, but because it does, i assume
it's meant to be the authoritative value of 'type'. so that's the field i grab
for my custom module, and when it's not kept up to date, stuff doesn't work.

i'm going to try to attach a patch, but last time i tried it crashed my browser
and i lost my post, so there may not be a patch file here.