twigphp/Twig · error · Twig\Error\RuntimeError

Attributes using InlineStyle can only be merged with…

Error message

Attributes using InlineStyle can only be merged with iterables or other InlineStyle instances.

What it means

InlineStyle::mergeInto merges this style object into a previous attribute value. A previous value that is neither iterable nor an InlineStyle instance cannot be combined, so a RuntimeError is thrown. This guards against overwriting a rendered string attribute with incompatible types.

Solutions

Ensure the previous value is an InlineStyle instance or an iterable of declarations.
Convert a plain CSS string into an iterable (explode on ';') before merging.
Initialize the attribute value as an empty InlineStyle instead of null/string.

Example fix

// before
$merged = $style->mergeInto('color:red');
// after
$merged = $style->mergeInto(new InlineStyle(['color' => 'red']));

Defensive patterns

Strategy: type-guard

Validate before calling

if (!$previous instanceof InlineStyle && !is_iterable($previous)) {
    throw new InvalidArgumentException('Cannot merge InlineStyle into non-iterable previous value');
}
$merged = $style->mergeInto($previous);

Type guard

function canMergeInlineStyle(mixed $previous): bool {
    return $previous instanceof InlineStyle || is_iterable($previous);
}

Try / catch

try {
    $merged = $style->mergeInto($previous);
} catch (\Twig\Error\RuntimeError $e) {
    if (str_contains($e->getMessage(), 'can only be merged')) {
        $merged = $style->mergeInto(new InlineStyle(is_iterable($previous) ? [...$previous] : []));
    } else {
        throw $e;
    }
}

Prevention

Keep attribute values as InlineStyle/iterables until final rendering
Never merge into already-rendered HTML strings
Initialize previous values as empty InlineStyle rather than null/string

When it happens

Trigger: Calling $inlineStyle->mergeInto('color:red') with a plain string, mergeInto(null), or merging into any scalar/object that is not InlineStyle or iterable.

Common situations: Merging into an attribute value that a template already rendered as a plain string, or into a null from an unset variable.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/a990ab7c86b8ac2e. Report an issue: GitHub.

Appendix: source

Thrown at extra/html-extra/HtmlAttr/InlineStyle.php:42

    {
        if (!is_iterable($value)) {
            throw new RuntimeError('InlineStyle can only be created from iterable values.');
        }

        $this->value = [...$value];
    }

    public function mergeInto(mixed $previous): mixed
    {
        if ($previous instanceof self) {
            return new self([...$previous->value, ...$this->value]);
        }

        if (is_iterable($previous)) {
            return new self([...$previous, ...$this->value]);
        }

        throw new RuntimeError('Attributes using InlineStyle can only be merged with iterables or other InlineStyle instances.');
    }

    public function appendFrom(mixed $newValue): mixed
    {
        if (!is_iterable($newValue)) {
            throw new RuntimeError('Only iterable values can be appended to InlineStyle.');
        }

        return new self([...$this->value, ...$newValue]);
    }

    public function getValue(): ?string
    {
        $style = '';
        foreach ($this->value as $name => $value) {
            // `0`, `0.0` and `'0'` are valid CSS values, so only the values that carry
            // no declaration at all are skipped here
            if (null === $value || false === $value || true === $value || '' === $value || [] === $value) {

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