Update with $unset fails with mapping exception

I'm using spring-data-mongodb 4.0.8. On creating an update with an $unset, the following is added to the document:

{  $unset: { fieldName: 1 } }

The value here - 1 - is then attempted to be mapped to the correct type for that field. In my case, that's Decimal128. I get this stack trace:

org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.Integer] to type [org.bson.types.Decimal128] for value [1]
 at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:47)
 at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:192)
 at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:175)
 at org.springframework.data.mongodb.core.convert.QueryMapper.applyFieldTargetTypeHintToValue(QueryMapper.java:848)
 at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedValue(QueryMapper.java:436)
 at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedObjectForField(QueryMapper.java:339)
 at org.springframework.data.mongodb.core.convert.UpdateMapper.getMappedObjectForField(UpdateMapper.java:160)
 at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedObject(QueryMapper.java:166)
 at org.springframework.data.mongodb.core.convert.UpdateMapper.getMappedObject(UpdateMapper.java:66)
 at org.springframework.data.mongodb.core.convert.QueryMapper.convertSimpleOrDocument(QueryMapper.java:578)
 at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedKeyword(QueryMapper.java:399)
 at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedObject(QueryMapper.java:146)
 at org.springframework.data.mongodb.core.convert.UpdateMapper.getMappedObject(UpdateMapper.java:66)
 at org.springframework.data.mongodb.core.QueryOperations$UpdateContext.getMappedUpdate(QueryOperations.java:864)
 at org.springframework.data.mongodb.core.MongoTemplate.doUpdate(MongoTemplate.java:1617)
 at org.springframework.data.mongodb.core.MongoTemplate.updateFirst(MongoTemplate.java:1540)

The value here does not relate to the field value, it should not be mapped. My current workaround is to override unset as below:

var update = new Update() {
    @Override
    public Update unset(String key) {
        addMultiFieldOperation("$unset", key, "");
        return this;
    }
};

The mapping does not fail with an empty string, and an empty string is what's used in the examples in the docs.