Can I use a Composite Key as criteria for selection in a query?

I have created a meeting minutes table and would like to use two fields as a unique ID for each meeting (field one is a meeting code (lookup from another table) for each type of meeting and field two is the meeting date - the combination would be unique for each record). An example would be ADHOC11/11/2015. I want to then set that composite key as the criteria for selecting the meeting to print a meeting minutes report. I was able to set both fields as the composite key, but it would not let me save the table due to the "primary key cannot be null" error. Is that because one of the fields was a lookup? I want the user to be able to select the type of meeting (code) from a drop down list, enter the meeting date, complete the remainder of meeting table fields and then be able to enter the meeting type and date combo to pull the minutes for that meeting when running the report. What is the best way to accomplish this goal?