The importmulti help text reads, in part:
{ (json object) If a ranged descriptor is used, this specifies the start and end of the range to import
"start": n, (numeric, optional, default=0) Start of the range to import
"end": n, (numeric, required) End of the range to import (inclusive)
},
This is a value for which a key is missing; it should be "range": { ...