PHP: Smarty: {foreach} doesn?t render my array

On one of the websites I work I couldn’t render an array in a smarty template. My code looked like this:

{foreach from=$myarray key=mykey item=items}

<h2>{$mykey}</h2>

{foreach from=$items item=it}

{$it.value}

{/foreach}

{/foreach}

I used print_r in my controller to show content of $myarray and it was just fine. After a while I found the error: Zend Framework did not allow me
to work [...]

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Source: http://de77.com/php/php-smarty-foreach-doesnt-render-my-array