Receiving "Invalid argument supplied for foreach() " when parsing XML data

Hi:
I receive "Invalid argument supplied for foreach()" when I am parsing an XML file.  The program goes ahead and parses out the data, but I need to understand the warning, then do what is needed to get rid of it.  My code is below.  It is the second 'foreach' that throws the warning (5th line).

Any help will be much appreciated!

foreach($data->STRUCTURES->ROOF as $roofingItem) {
      $count++;
}
while($i <= $count) {
      foreach($data->STRUCTURES->ROOF[$i]->LINES->LINE as $lineitem) {
            $type=$data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['type'];
            if($type == "VALLEY") {
                  $valley=$valley + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "RIDGE") {
                  $ridge=$ridge + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "Eave") {
                  $eave=$eave + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "Rake") {
                  $rake=$rake + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "Flashing") {
                  $flashing=$flashing + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "StepFlash") {
                  $stepflash=$stepflash + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            if($type == "HIP") {
                  $hip=$hip + $data->STRUCTURES->ROOF[$i]->LINES->LINE[$ii]['length'];
            }
            $ii++;
      }