Incorrect column conditional formula

Hi, I'm quite new to sharepoint.

I have 2 columns in my sharepoint list:

- Intermediary type (Choice)

- Intermediary type: Others (Free text)

In my form, I want the 2nd column to only appear if in the 1st column, 'Others' is selected.

In my conditional formula box, I put my formula as 

=IF([$Intermediary type] == 'Others', 'true', 'false')

But it says "Enter a valid condition"

Please help.

Thank you.