htmlspecialchars() error (w/ backtrace)

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Warning: htmlspecialchars() [<a href='function.htmlspecialchars'>function.htmlspecialchars</a>]: Invalid multibyte sequence in argument in /home/benbra/CafeNebulus/piwik/core/DataTable/Filter/SafeDecodeLabel.php on line 53

Backtrace -->

#0 Piwik_ErrorHandler(...) called at [:]
#1 htmlspecialchars(...) called at [/home/benbra/CafeNebulus/piwik/core/DataTable/Filter/SafeDecodeLabel.php:53]
#2 Piwik_DataTable_Filter_SafeDecodeLabel::safeDecodeLabel(...) called at [/home/benbra/CafeNebulus/piwik/core/DataTable/Filter/SafeDecodeLabel.php:70]
#3 Piwik_DataTable_Filter_SafeDecodeLabel->filter(...) called at [/home/benbra/CafeNebulus/piwik/core/DataTable.php:387]
#4 Piwik_DataTable->filter(...) called at [/home/benbra/CafeNebulus/piwik/core/DataTable.php:414]
#5 Piwik_DataTable->applyQueuedFilters(...) called at [/home/benbra/CafeNebulus/piwik/core/API/ResponseBuilder.php:300]
#6 Piwik_API_ResponseBuilder->handleDataTable(...) called at [/home/benbra/CafeNebulus/piwik/core/API/ResponseBuilder.php:78]
#7 Piwik_API_ResponseBuilder->getResponse(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator.php:142]
#8 Piwik_API_DataTableManipulator->loadSubtable(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:103]
#9 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#10 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#11 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#12 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#13 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#14 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:124]
#15 Piwik_API_DataTableManipulator_Flattener->flattenRow(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:69]
#16 Piwik_API_DataTableManipulator_Flattener->doManipulate(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator.php:85]
#17 Piwik_API_DataTableManipulator->manipulate(...) called at [/home/benbra/CafeNebulus/piwik/core/API/DataTableManipulator/Flattener.php:53]
#18 Piwik_API_DataTableManipulator_Flattener->flatten(...) called at [/home/benbra/CafeNebulus/piwik/core/API/ResponseBuilder.php:284]
#19 Piwik_API_ResponseBuilder->handleDataTable(...) called at [/home/benbra/CafeNebulus/piwik/core/API/ResponseBuilder.php:78]
#20 Piwik_API_ResponseBuilder->getResponse(...) called at [/home/benbra/CafeNebulus/piwik/core/API/Request.php:129]
#21 Piwik_API_Request->process(...) called at [/home/benbra/CafeNebulus/piwik/core/ViewDataTable.php:431]
#22 Piwik_ViewDataTable->loadDataTableFromAPI(...) called at [/home/benbra/CafeNebulus/piwik/core/ViewDataTable/HtmlTable.php:80]
#23 Piwik_ViewDataTable_HtmlTable->main(...) called at [/home/benbra/CafeNebulus/piwik/plugins/Actions/Controller.php:464]
#24 Piwik_Actions_Controller->configureGenericViewActions(...) called at [/home/benbra/CafeNebulus/piwik/plugins/Actions/Controller.php:406]
#25 Piwik_Actions_Controller->configureViewActions(...) called at [/home/benbra/CafeNebulus/piwik/plugins/Actions/Controller.php:46]
#26 Piwik_Actions_Controller->getPageUrls(...) called at [:]
#27 call_user_func_array(...) called at [/home/benbra/CafeNebulus/piwik/core/FrontController.php:136]
#28 Piwik_FrontController->dispatch(...) called at [/home/benbra/CafeNebulus/piwik/index.php:52]