$ cat spack.yaml
spack:
  definitions:
    - first: [libelf, libdwarf]
    - compilers: ['%gcc@4.8.5']
    - second:
      - $first
      - matrix:
        - [zlib]
        - [$compilers]
  specs:
    - $second
    - cmake

$ spack concretize
==> Error: Invalid character

$ spack --debug concretize
...
==> [2019-07-23-09:09:34.269444] LexError: Invalid character
==> [2019-07-23-09:09:34.269503] Error: Invalid character
Traceback (most recent call last):
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/main.py", line 697, in main
    return _invoke_command(command, parser, args, unknown)
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/main.py", line 447, in _invoke_command
    return_val = command(parser, args)
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/cmd/concretize.py", line 22, in concretize
    env.write()
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/environment.py", line 1242, in write
    Spec(s) in speclist.specs]
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/spec.py", line 949, in __init__
    spec_list = SpecParser(self).parse(spec_like)
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/parse.py", line 151, in parse
    self.setup(text)
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/parse.py", line 148, in setup
    self.push_tokens(self.lexer.lex(text))
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/parse.py", line 86, in lex
    tokens = self.lex_word(word)
  File "/Users/pbryant/Development/40-ECP/spack/lib/spack/spack/parse.py", line 79, in lex_word
    raise LexError("Invalid character", word, word.index(remainder))
spack.parse.LexError: Invalid character