Calculate the median using a sharepoint liste as database

Hello all,

I need to calculate the median for a column from my database that I imported as a Sharepoint list. Therfoe I used this formula: 

Median Number of Invoicing = MEDIAN('Digital_Benchmarking_Database_pilot '[Number of invoicins])

But I always get the error message that MEDIAN does not support expreesions from the type String/Boolean/Date.

Can anyone help me with that?

Thanks in advance and best,

Joh