how to take a digest of a ranger object in R

I want a checksum on a ranger object. (random forest)
Can I do that in memory, without writing to a file?

Here is my dummy code:

library(digest)
digest(my_ranger_object, algo = "sha1", serialize = T)

I get the following error:

Error in serialize(object, connection = NULL, ascii = ascii, version = serializeVersion) : 
  object 'my_ranger_object' not found

How do I make this work?