[Bug] Cannot resolve string concatenation like '[xxx]'

[TIIEHenry]

{% if messages[0]['role'] == 'system' %}
{% set loop_messages = messages[1:] %}
{% set system_message = messages[0]['content'] %}
{% elif USE_DEFAULT_PROMPT == true and not '<>' in messages[0]['content'] %}
{% set loop_messages = messages %}
{% set system_message = 'DEFAULT_SYSTEM_MESSAGE' %}
{% else %}
{% set loop_messages = messages %}
{% set system_message = false %}
{% endif %}
{% for message in loop_messages %}
{% if (message['role'] == 'user') != (loop.index0 % 2 == 0) %}
{{ raise_exception('Conversation roles must alternate user/assistant/user/assistant/...') }}
{% endif %}
{% if loop.index0 == 0 and system_message != false %}
{% set content = '<>\n' + system_message + '\n<>\n\n' + message['content'] %}
{% else %}
{% set content = message['content'] %}
{% endif %}
{% if message['role'] == 'user' %}
{{ bos_token + '[INST] ' + content.strip() + ' [/INST]' }}
{% elif message['role'] == 'system' %}
{{ '<>\n' + content.strip() + '\n<>\n\n' }}
{% elif message['role'] == 'assistant' %}
{{ ' ' + content.strip() + ' ' + eos_token }}
{% endif %}
{% endfor %}
[TemplateError{severity=FATAL, reason=SYNTAX_ERROR, item=OTHER, message='java.beans.IntrospectionException: Method not found: getChars', fieldName='bos_token + '[INST] ' + content.strip() + ' [/INST]'', lineno=21, startPosition=-1, scopeDepth=2, category=UNKNOWN, categoryErrors=null}, TemplateError{severity=FATAL, reason=SYNTAX_ERROR, item=OTHER, message='java.beans.IntrospectionException: Method not found: getChars', fieldName='' ' + content.strip() + ' ' + eos_token', lineno=25, startPosition=-1, scopeDepth=2, category=UNKNOWN, categoryErrors=null}]