I received a warning flag that PMID ids need to be 6 digits, but the paper I was storing does only have four:
Thanks for reporting @anna. I have let the team know about the error message that appears in the PMID field of the metadata edit dialog.
How did you initially encounter the error? For example, did you try to save the reference by entering its details manually? When I use the Paperpile button to directly save the reference from the webpage, the reference and its 4 digit PMID, automatically saves to my library:
