I created a custom AcceptedValueMinimum class derived from AcceptableValueBase which has only a minimum value restriction. ConfigurationManager will then treat this custom class as AcceptableValueRange, because it does not have the property AcceptableValues present in AcceptableValueList. This will always throw an ArgumentNullException because ConfigurationManager checks for the existence of both MinValue and MaxValue properties in the custom class, which are not present in this case.

PS: Thanks in advance for your help!