gglresearchanddevelopment/ironlanguages-main — comment on commit 630995eca293b5b0531e821b76bab7e15c4506f6
File: Languages/Ruby/Libraries/Builtins/Marshal.cs line 457
By tmat:

Exception objects don't seem to have instance data ('I') prefix - they have 'o':

"\x04\bo:\x0EException\a:\tmesgI\"\nhello\x06:\x06EF:\abt[\aI\"\x06A\x06;\aFI\"\x06B\x06;\aF"