COUNTX() error

Hi experts,

I'm scratching my head about this trivial problem:

This doesn't work:

=COUNTX(ALL(Table1[Code]), Table1[Sales Amount])

Error msg:

A single value for column 'Sales Amount' in table 'Table1' cannot be determined. This can happen when a measure formula refers to a column that contains many values without specifying an aggregation such as min, max, count, or sum to get a single result.'.

However the error msg doesn't make sense, COUNTX() is supposed to take a set of values instead of a single value as second parameter, and if anything is wrong, it's probably ALL(), because this works:

=COUNTX(ALL(Table1), Table1[Sales Amount])

Question: Why the first one doesn't work?