"Undistinguishable" vs. "indistinguishable"

I'm a native English speaker, and I've never heard of "undistinguishable". I searched for undistinguishable and Google replied with:

Did you mean: indistinguishable

Princeton University's WordNet defines indistinguishable as:

• identical: exactly alike; incapable of being perceived as different; "rows of identical houses"; "cars identical except for their license plates"; "they wore indistinguishable hats"

• not capable of being distinguished or differentiated; "the two specimens are actually different from each other but the differences are almost indistinguishable"; "the twins were indistinguishable"; "a colorless person quite indistinguishable from the colorless mass of humanity"

To convey the sense of "you can't tell what it is", you could use indecipherable or inscrutable.

I've never seen "undistinguishable" before. My spell-check flags it as an error and suggests "indistinguishable". I suspect it's a typo or a case of misspelling a word in a logical way. I can't imagine that its meaning would be different from "indistinguishable". The Corpus of Contemporary American English (COCA) lists only 8 hits for "undistinguishable" and 1000+ hits for "indistinguishable". I'd stick to the latter.