Hi,
A code-block can be made by using the
icon when creating a post.
This will create the following:

Paste the content of the file between the tickboxes (replacing the line “type or paste your code here”)
As example :

The result (see below) is the shiney markup from my test-csv-file.
I have tried to recreate the issue which you are facing, and made the following .csv-file:
testimport01,192.168.1.201
testimport02,192.168.1.202
testimport03,192.168.1.203
testimport04,192.168.1.204
testimport05,192.168.1.205
testimport06,192.168.1.206
testimport07,192.168.1.207
testimport08,192.168.1.208
testimport09,192.168.1.209
testimport10,192.168.1.210
Then offered/uploaded it to my CRE 2.1.0p37 setup via Upload.
By default the checkbox for “has title line” was on, but as you see above i did not make one, so the 1st line was not seen in the preview, unticked it, clicked Update Preview, and the 1st line is also there.
Then selected the correct column-name for both the hostname and the ipv4 address from the drop-down.
So this scenario did not produce the error.
Went a bit further by adding a header line with column-names:
Hostname,IPv4 address
testimport01,192.168.1.201
testimport02,192.168.1.202
testimport03,192.168.1.203
testimport04,192.168.1.204
testimport05,192.168.1.205
testimport06,192.168.1.206
testimport07,192.168.1.207
testimport08,192.168.1.208
testimport09,192.168.1.209
testimport10,192.168.1.210
Again did the import, left option has title line checked, this was accepted without getting the error you have seen.
I tried the same with pasting the content directly, and (both cases) i was unable to reproduce your error.
So as @r.sander suggested, please share your csv-file in the post.