Mystery foreach error

I've been getting the following error for awhile now, but I can't figure out why it's happening: Invalid argument supplied for foreach() in ... sample.php on line 377 Here's that portion of code: include("states_zipcodes.php"); // Check if Zip Code matches from states_zipcodes $zip_short = substr($zip, 0, 3); foreach ($states[$state] as &$zip_prefix) { // <-- line 377 if ($zip_prefix == $zip_short) { break; } else { $match = 'no'; } } It doesn't happen all the time, so I'm thinking that some spambot is populating the HTML form with something the script doesn't like(?). However, I tested that myself by entering text, or by entering just 2 digits, but there was no error. FYI, I do have code in the script that catches faulty input and warns people in their browser to go back and re-enter the correct data, so I'm at a loss as to why this is happening. How can I see what's triggering this to happen? I have the following line in my php.ini: error_reporting = E_ALL & E_STRICT & E_NOTICE & E_DEPRECATED Thank you, Angela