NULL character found in serialized string

I am unable to unserialize() a backtrace that I serialize()'d and saved to a text file. I cannot also copy-paste the string, it only copies the data before NULL so I am unable to unserialize it back in PHP.

Here's a small excerpt of what it looks like in Notepad++

Any ideas how to unserialize this?

Note: I am using error_log($backtrace, 3, 'file.log'); to save the file, then simply open in Notepad++ to copy-paste but I can't copy past the NULL character.