Server can't handle an annotation with `id=null`

[leonardr]

Example:

{
"body":{
"http://librarysimplified.org/terms/progressWithinBook":0.22,
"http://librarysimplified.org/terms/progressWithinChapter":0.33846155,
"http://librarysimplified.org/terms/chapter":"Alice in Wonderland",
"http://librarysimplified.org/terms/device":"null",
"http://librarysimplified.org/terms/time":"2018-04-25T18:00:05.290Z"
},
"id":null,
"motivation":"http://www.w3.org/ns/oa#bookmarking",
"target":{
"selector":{
"type":"oa:FragmentSelector",
"value":"{"idref":"item25","contentCFI":"/4/110[Page_7]/2,/1:0,/1:1"}"
},
"source":"http://www.gutenberg.org/ebooks/35688"
},
"type":"Annotation",
"@context":"http://www.w3.org/ns/anno.jsonld"
}

"A key-value pair in the body of a JSON-LD document whose value is null has the same meaning as if the key-value pair was not defined." -- https://www.w3.org/TR/json-ld-syntax/#general-terminology

I don't know what the exact error is but it should be easy to duplicate this problem.