Power Query: Always getting DataFormat.Error We couldn't convert to Number

I'm new to using Power Query and I seem to always get error:DataFormat.Error We couldn't convert to Number. This seems to happen when it really shouldn't so I'd be grateful for an explanation. For example:

I have the following data:

As can be seen, dep_delay is a column of numbers and carrier is a text column. If I try to Pivot on carrier, with dep_delay as value and aggregation function Average, I get a DataFormat.Error We couldn't convert to Number. This is non-sensical to me - everything that needs to be is already of type number.  I get the same error if I try to close and load the query.

Worth noting is that, if I simply import the Table from csv using get and transform - but not the PQ editor - the data loads correctly and I can do this pivot etc. in Excel with absolutely no issues - suggesting something strange is going on in PQ. (Perhaps worth mentioning is that I'm using PQ on a mac).

Grateful for any suggestions as to why this might be occuring.