Is there a verb for singing nonsense syllables to substitute for unknown lyrics?

Someone who doesn't know all the lyrics to a song might sing "la la la" or "dum dum dum" in place of the verses they don't know. Is there a verb for this? Similar to "humming" the tune, but with syllables?

I'm aware of the verb "scat," but to me it implies a particular style of fast-paced jazz nonsense vocalization. I'm looking for a more general word, if it exists.

You can say fudge the lyrics.

Example of authentic use:

I discovered recently that there's a line that goes "Just your typical prototype." I usually fudged my way through the lyrics with something having to do with keeping kimonos tied, and wondered why the hell her bathrobe had anything to do with it.

Instrumentalists do this too, but then it's called faking it. You have to do that in certain passages of the last movement of Tchaikovsky's Fourth Symphony.