It is not erroneous. *Mi estas malvarma* and *Mi malvarmas* are acceptable translations. (You will find that the sentence *Ĉu vi estas varmaj?* appears in Engholm's *Infanoj en Torento*.)

However, some people dislike it and consider it ambiguous: it might mean "I am a cold person" or "I am hypothermic" or something like that. (In particular, *Ich bin kalt/heiss* is inappropriate in German, and so native German-speakers are most likely to be bothered by it.) Therefore, alternative translations are:

- *Mi sentas min malvarma.* I feel [myself to be] cold.

- *Mi sentas malvarme.* I feel cold[ly].

- *Al mi estas malvarme. Al mi malvarmas.* It is cold for me.