Exception running workflow - 'ValueError: malformed node or string: <_ast.BinOp object at 0x7f80d29946d8>'

I haven't been able to find where this error is being generated from. As far as I can tell the action follows format and syntax correctly.

Action from workflow that is failing

  send_maintenance_start:
    action: email.send_email
    input:
      account: smarthost
      email_from: "no-reply@XXXXXX"
      email_to: "carlos@XXXXXX"
      subject: Incident detected on <% ctx("hostname") %>
      message: Automatic reboot of <% ctx("hostname") %> to recover from host down.
    next:
      - do: disable_users

Exception from workflowegine log.

2020-01-17 23:33:43,532 140191264317064 INFO (unknown file) [-] [5e22445438b7e59d28ade4fb] Mark task "send_maintenance_start", route "0", in conductor as running.
2020-01-17 23:33:43,580 140191264317064 INFO (unknown file) [-] [5e22445438b7e59d28ade4fb] Requesting execution for task "send_maintenance_start", route "0".
2020-01-17 23:33:43,580 140191264317064 INFO (unknown file) [-] [5e22445438b7e59d28ade4fb] Processing task execution request for task "send_maintenance_start", route "0".
2020-01-17 23:33:43,589 140191264317064 INFO (unknown file) [-] [5e22445438b7e59d28ade4fb] Task execution "5e22445738b7e59c9835e3ff" created for task "send_maintenance_start
", route "0".
2020-01-17 23:33:43,599 140191264317064 WARNING (unknown file) [-] Determining if exception <class 'ValueError'> should be retried.
2020-01-17 23:33:43,599 140191264317064 WARNING (unknown file) [-] Determining if exception <class 'ValueError'> should be retried.
2020-01-17 23:33:43,599 140191264317064 ERROR (unknown file) [-] [5e22445438b7e59d28ade4fb] Failed action execution(s) for task "send_maintenance_start", route "0". malforme
d node or string: <_ast.BinOp object at 0x7f80d29946d8>
Traceback (most recent call last):
  File "/opt/stackstorm/st2/lib/python3.6/site-packages/st2common/util/casts.py", line 36, in _cast_object
    return json.loads(x)
  File "/usr/lib/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Start with st2common/util/casts.py, especially _cast_object, and compare its JSON parsing path with the shown workflow action and execution log. Trace how the subject and message expressions are converted before email.send_email runs. Done means the source of the malformed AST value is identified and the failing workflow input is handled or clearly rejected.