1. The sentence doesn't contain a grammatical error.
2. "Passive voice" not "passive tense":
- Tense = time-verb relationship
- Voice = subject-verb relationship
3. The "passive voice" is not a grammatical error. It's a grammatical construction.
4. The "be" in this sentence is a linking verb. It links the subject with the adjective "real". No action takes place at all.
The word "to be" can have three functions:
a) Rarely: main verb, meaning "behave":
- Stop being silly.
b) Linking verb: Linking the subject with one of its attributes.
- The cat is black.
c) An auxilary verb. Used to indicate a grammatical function. One of the grammatical functions "be" can indicate is the passive voice:
- The car was stolen.
Another grammatical function: the progressive aspect:
- He was playing the piano (when the lightning struck).
5. The "passive voice" doesn't need a form of "to be", either:
- They got married last Sunday.