multi-part identifier could not be bound

Hi, I'm having an issue with my SQL Statement. I'm trying to declare a variable and set it to the count of the number of records that exist for a particular order number. The problem is when I execute the statement I get this error:

Msg 4104, Level 16, State 1, Line 3
The multi-part identifier "prod.dbo.HEADERS.ORDER_NUMBER" could not be bound.

I'm 100% Sure everything is spelled correctly and capitalized properly. This is the code I'm trying to run:

Code:

DECLARE @test INT SET @test = (SELECT COUNT(*) FROM [prod].[dbo].[LINES] WHERE [ORDER_NUMBER] = [prod].[dbo].[HEADERS].[ORDER_NUMBER])

Any idea what might be causing this? Thank you.