Unsupported operand types: string + int

Trace:

<p>#0 discuz_error::debug_backtrace called at discuz_error.php:178</p><p>#1 discuz_error::exception_error called at class_core.php:121</p><p style="margin-left:20px;">Args: TypeError</p><p>#2 core::handleException called at :</p><p style="margin-left:20px;">Args: TypeError</p>