Writing to S3 bucket from Lambda, same account, “InvalidARN" message

Been digging through tutorials for days, but they all say the same thing, and it seems like I should be in slam dunk territory here, but I get the following error whenever I try to read or write from my S3 bucket.

“InvalidARN: ARN accountID does not match regex ”[0-9]{12}“”

I only have one AWS account, so my lambda function should be owned by the same account as my S3 bucket. I have given my lambda role s3:GetObject and PutObject permissions, as well as just s3:*, I have verified that my S3 bucket policy is not denying access explicitly, but nothing changes the message.

I am new to AWS policies and permissions, but google isn't giving up a lot of other people getting this message. I don't know where I am supposed to be supplying my AccountID or why it isn't already there. Would be grateful for any insights.