First time a read this I thought it was shield (Still not entirely sure, either me or unit is horribly confused)
If it is:
Way UP, Fire shield costs 4 upped and will always deal 1 damage, this will rarely be better as by the time it will statistically poison twice fire shield will already have gotten 4 damage done. This may be better in the long run though, if fire buckler is stolen/destroyed then this will have the creatures still taking damage.
If it's not:
Nice idea. (With a reduced chance)