One query runs fine but a very similar one does not run at all

Question

I have been using SQL for many years, but I am pretty new to postgresql. I'm using pgAdmin and I'm running the following query just fine.

select *
from budget_spending
where market is not null
limit 10

However, this query does NOT run.

select *
from budget_spending
where Project_Description is not null
limit 10

I get this error message:

ERROR:  column "project_description" does not exist

This is very bizarre, because I'm looking at it and it certainly does exist. If I right-click the column and go to Properties, I see 'Project_Description' as the name of the column. What am I missing here?