I guess it might be because it is expecting a single value?
taken from the animate page on jQuery:
All animated properties should be animated to a single numeric value, except as noted below; most properties that are non-numeric cannot be animated using basic jQuery functionality. (For example, width, height, or left can be animated but background-color cannot be.) Property values are treated as a number of pixels unless otherwise specified. The units em and % can be specified where applicable.