twigphp/Twig · error · SyntaxError

You cannot assign a value to

Error message

You cannot assign a value to "%s".

What it means

Twig raises this SyntaxError at compile time when a template tries to assign to a reserved literal name: true, false, none, or null (case-insensitive). These tokens are constants/literals in Twig, not variables, so they can never be assignment targets. AssignNameExpression's constructor rejects them before the template is compiled.

Solutions

1. Rename the assignment target in the template to a valid variable name (e.g. `isTrue`, `isNull`).
2. If the value should be a literal, drop the assignment and use `true`/`false`/`none`/`null` directly in expressions.
3. In PHP code building nodes, validate the name against ['true','false','none','null'] before constructing AssignNameExpression.

Example fix

// before (template)
{% set true = user.isActive %}
// after
{% set isActive = user.isActive %}

$reserved = ['true', 'false', 'none', 'null'];
if (in_array(strtolower($name), $reserved, true)) {
    throw new \InvalidArgumentException("'$name' is a Twig reserved word and cannot be a variable name");
}

When it happens

Trigger: Instantiating AssignNameExpression (or compiling a template containing) `{% set true = ... %}`, `{% set FALSE = ... %}`, `{% set null = x %}`, `{% for none in items %}`, or macro/import blocks targeting those names; also direct PHP construction `new AssignNameExpression('true', $lineno)`.

Common situations: Typos like `{% set false = someVar %}` when meaning to test a value; developers porting from languages where `true`/`null` are valid identifiers; generated templates that build `set` targets from unvalidated user input.

Related errors

• Cannot assign to " ", only variables can be assigned in…
• Cannot assign to " ", only variables can be assigned in…
• Positional arguments cannot be used after named arguments…
• Argument " " is defined twice for " ".
• An exception has been thrown during the compilation of a…

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/a4c6efb081d8ad42. Report an issue: GitHub.

Appendix: source

Thrown at src/Node/Expression/AssignNameExpression.php:30

namespace Twig\Node\Expression;

use Twig\Compiler;
use Twig\Error\SyntaxError;
use Twig\Node\Expression\Variable\AssignContextVariable;
use Twig\Node\Expression\Variable\ContextVariable;

class AssignNameExpression extends ContextVariable
{
    public function __construct(string $name, int $lineno)
    {
        if (self::class === static::class) {
            trigger_deprecation('twig/twig', '3.15', 'The "%s" class is deprecated, use "%s" instead.', self::class, AssignContextVariable::class);
        }

        // All names supported by ExpressionParser::parsePrimaryExpression() should be excluded
        if (\in_array(strtolower($name), ['true', 'false', 'none', 'null'], true)) {
            throw new SyntaxError(\sprintf('You cannot assign a value to "%s".', $name), $lineno);
        }

        parent::__construct($name, $lineno);
    }

    public function compile(Compiler $compiler): void
    {
        $compiler
            ->raw('$context[')
            ->string($this->getAttribute('name'))
            ->raw(']')
        ;
    }
}

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