Unable to specify both source and size with New-VHD command

Hi all!  I'm working with Hyper-V Server 2012 R2 (fully patched).

new-vhd -Path c:\util.vhdx -Fixed -SourceDisk 3 -SizeBytes 50GB fails with the errorNew-VHD : Parameter set cannot be resolved using the specified named parameters.

However, if I run the command above without either the SourceDisk or SizeBytes parameter it succeeds.  It only fails when they're both present.  TechNet documentation shows both parameters in the same command.  Does anyone know what I'm doing wrong?