While evaluating the form starting at line 36, column 0
  of #P"/home/user/quicklisp/local-projects/MBuild/Mezzano/../build-cold-image.lisp":

debugger invoked on a SIMPLE-ERROR in thread
#<THREAD "main thread" RUNNING {1000510083}>:
  The assertion (MEZZANO.LAP.ARM64::SHIFTED-MASK-P MEZZANO.LAP.ARM64::IMM)
  failed with MEZZANO.LAP.ARM64::IMM = 220.

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE] Retry assertion.
  1: [RETRY   ] Retry EVAL of current toplevel form.
  2:            Ignore error and continue loading file "/home/user/quicklisp/local-projects/MBuild/Mezzano/../build-cold-image.lisp".
  3: [ABORT   ] Abort loading file "/home/user/quicklisp/local-projects/MBuild/Mezzano/../build-cold-image.lisp".
  4:            Ignore runtime option --load "../build-cold-image.lisp".
  5:            Skip rest of --eval and --load options.
  6:            Skip to toplevel READ/EVAL/PRINT loop.
  7: [EXIT    ] Exit SBCL (calling #'EXIT, killing the process).

(SB-KERNEL:ASSERT-ERROR (MEZZANO.LAP.ARM64::SHIFTED-MASK-P MEZZANO.LAP.ARM64::IMM) 1 MEZZANO.LAP.ARM64::IMM 220)