[Solved] type of key

[strato]

what type of "key" is in this structure?

foreach myarray as key => value

in my case line 2 causes an "Error data type mismatch" error:

foreach myarray as key => value
     c = lower(key)

queried in the troubleshooter, the key content is the uppercase name of the first item, and:

? type("key")
C

what can cause such an error?