Just to note for future, saying something "does not work" does not provide any information. In this instance, "does not work" means that you have data where one or the other result is missing, when using + a missing in either result means the output will be . (missing). This does not mean it is not working, but just not working as you expect it to, its all in the documentation. In this instance the sum() function ignores missing values, and returns either the one number, or the sum of both of them - again per documentation.
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