Morphia throws error when the field was changed its type from String to enum: No enum constant

In DAO class I use with Morphia I have replaced the String filed type to enums.Currency and I expect that everything will work fine, when the data is retrieved from the db:

was

    public String currency;

now

    public Currency currency;

where Currency is:

public enum Currency {
    EUR, USD, RUB
}

I expect that nothing should change, however, I'm getting the error message like that:

Caused by: java.util.concurrent.CompletionException: java.lang.RuntimeException: java.lang.IllegalArgumentException: No enum constant enums.Currency.
        at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
        at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
        at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1773)
        ... 6 common frames omitted
Caused by: java.lang.RuntimeException: java.lang.IllegalArgumentException: No enum constant enums.Currency.
        at dev.morphia.mapping.EmbeddedMapper.fromDBObject(EmbeddedMapper.java:84)
        at dev.morphia.mapping.Mapper.readMappedField(Mapper.java:906)
        at dev.morphia.mapping.Mapper.fromDb(Mapper.java:308)
        at dev.morphia.mapping.Mapper.fromDBObject(Mapper.java:215)
        at dev.morphia.query.internal.MorphiaCursor.next(MorphiaCursor.java:87)
        at dev.morphia.query.internal.MorphiaCursor.toList(MorphiaCursor.java:57)
         ... 6 common frames omitted
Caused by: java.lang.IllegalArgumentException: No enum constant enums.Currency.
        at java.base/java.lang.Enum.valueOf(Enum.java:264)
        at dev.morphia.converters.EnumConverter.decode(EnumConverter.java:19)
        at dev.morphia.converters.Converters.fromDBObject(Converters.java:127)
        at dev.morphia.mapping.ValueMapper.fromDBObject(ValueMapper.java:19)
        at dev.morphia.mapping.Mapper.readMappedField(Mapper.java:906)
        at dev.morphia.mapping.Mapper.fromDb(Mapper.java:308)
        at dev.morphia.mapping.EmbeddedMapper.fromDBObject(EmbeddedMapper.java:73)
        ... 35 common frames omitted

What would be the reason and why string values are not just interpreted as enum values?

This error is raised when there are String values in the Mongodb not existing in the enum.

Here I have realized there are db entries with empty string fields:

After replacing the empty string values in the Mongo Db to those with one of enum, the error has gone.