Gotcha: db index name must be a string

Might just be Rails 2.3.8 - an exception is thrown whenever I use a symbol for an index name. I get:

rake aborted!
An error has occurred, all later migrations canceled:
undefined method `length' for :my_index_name

The code causing this would be something like:

add_index :widgets, [:colour, :weight], :name => :index_by_colour_weight

it’s solved by just using a string instead:

add_index :widgets, [:colour, :weight], :name => 'index_by_colour_weight'