Extra comma in APA style: letter or memorandum

APA style generally dictates that a letter or memorandum be cited in the text, but not included in the list of references. However, at our agency, we typically do list technical memos in our reference list. When I include such a memo in an APA-style list of references, I get an extra comma and space after the word "memorandum" as shown below:

Smith, J. (2011, December 21). Nomenclature derivatives for a new economy. Memorandum, .

Here is the data in Zotero:

Type
Letter

Author
John Smith

Recipient
Kari Zimmerman

Type
Memorandum

Date
2011-12-21

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