Composite keys

Hello everyone.

I'm trying to put together a composite key between two fields from the query editor. But it returns error, and I am using the correct syntax.

For example, I have two fields "date" and "market".

Formula: [Fecha] & " " & [Market]

Expression: = Table.AddColumn(#"Columnas quitadas1", "Key", each [Fecha] & " " & [Market])

Expression.Error: The & operator can not be applied to the Date and Text types.

Details:
Operator=&
Left=1/4/2018
Right=

How can I join two fields of different types for the composite key?

Thanks!