After updating Npgsql, Marten started to throw exceptions:
[NullReferenceException: Object reference not set to an instance of an object.]
   Marten.Util.TypeMappings.ToDbType(Type type) +93
   Marten.Schema.DuplicatedField..ctor(EnumStorage enumStorage, MemberInfo[] memberPath) +141
   Marten.Schema.DocumentMapping.DuplicateField(MemberInfo[] members, String pgType, String columnName) +195
   Marten.Events.EventQueryMapping.duplicateField(Expression1 property, String columnName) +94
   Marten.Events.EventQueryMapping..ctor(StoreOptions storeOptions) +485
   Marten.Storage.StorageFeatures.PostProcessConfiguration() +364
   Marten.DocumentStore..ctor(StoreOptions options) +259
   Marten.DocumentStore.For(Action1 configure) +60A quick search through the issues list on Github, brought me to the following page: https://github.com/JasperFx/marten/issues/1044
Good news, the issue is already resolved and closed in Marten v3(alpha 2 version is available).