IF() Syntax problem

IF() Syntax problem

Hi, I'm a little stuck setting up a formula for a custom field and hope someone can help

I want to make an IF formula field based on the contents of 2 other custom fields;

1. a picklist field named Premium Frequency with options of Monthly & Annual
2. a currency field named Premium to have a manually entered value

I want the new field to show the annual premium - I came up with the following but I get an error message saying 'Error. Missing Quotes'

If(${Products.Premium Frequency}=='Monthly',${Products.Premium}*12,${Products.Premium})

can someone let me know where I'm going wrong?

thanks