Why, "Expected version between 2.1 and 2.dev but got 1.14"

When trying to upload a DAR to a participant, I see the following error:

upload-dar did not succeed:
"ALLOWED_LANGUAGE_VERSIONS(8,a276e426):
Disallowed language version in package 86828...:
Expected version between 2.1 and 2.dev but got 1.14" 

What could cause this?

In this particular case, I was doing the following:

• Uploading a 2.x-compiled DAR
  (language version 1.14)
• Into a 3.x participant node
  (which supports language version 2.1 or 2.dev)

Also see the ALLOWED_LANGUAGE_VERSIONS docs.