php merge_array表示JSON数据不是数组

Question

Having an issue here. When I attempt to do this:

foreach ($validation_fields as $val) {
        $rule = json_decode($val['rules'], true);
        $key = 'fields[' . $val['group_id'] . '][' . $val['field_id'] . ']';
        if (array_key_exists($key, $rules)) {
        array_merge($rules[$key], $rule);
        }
    }

PHP throws an error back at me saying that argument 2 is not an array, however, when I dump $rule or use is_array($rule), it checks out as being an array. I am so confused.

the json I am decoding is:

'{"format": {"rule": ["matches","/[0-9]{9}/"], "negate": true,"message": "Apps"}}'

var_dump(json_decode($rule)) is:

array
  'format' => 
    array
      'rule' => 
        array
          0 => string 'matches' (length=7)
          1 => string '/[0-9]{9}/' (length=10)
      'negate' => boolean true
      'message' => string 'Apps' (length=30)

array_merge() [function.array-merge]: Argument #2 is not an array on line 197 in ....