I was trying to work ahead to support v4.0.0 and RetrievePagePropertyItem and noticed it was already in v3.1.0 but got this error when trying to retrieve a number property on a simple 5 property database. Title, Date, Select, and RichText type properties work fine.

Newtonsoft.Json.JsonSerializationException
Error converting value 6043 to type 'Notion.Client.Number'. Path 'number', line 1, position 71.
Source=Newtonsoft.Json
Inner Exception 1:
ArgumentException: Could not cast or convert from System.Int64 to Notion.Client.Number.