Update Query Doesnt work?

These are my table structures

tblcomment

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comment_i开发者_高级运维d

content_id

user_id

picture_id

comment

comment_time

tblpictures

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content_id

title

picture_id

I am trying to update the content_id field on pictures using this but it doesnt seem to work ?

UPDATE `comments`
SET 
comments.content_id = pictures.content_id
WHERE
comments.picture_id = pictures.picture_id

I get this error

1054 - Unknown column 'pictures.picture_id' in 'where clause'

UPDATE comments, pictures SET ...