I am learning German with our lord and savior Duolingo and came across this sentence:

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> The cheese is good although it is old.
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... which I translated as...

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> Der Käse ist gut, obwohl **es** alt ist.
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... and Duo said that is wrong, and that the correct answer is:

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> Der Käse ist gut, obwohl **er** alt ist.
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I think "der Käse" should be an "it/es", not a "he/er". Why is "es" wrong and "er" correct here?