Why is PHP mysqli prepared statement working but inserting all NULL values?

Question

What would cause this? Code follows:

$m = new mysqli($host,$user,$pass,$db);
if(mysqli_connect_errno()) die('Connect failed: ' . mysqli_connect_error());
$PrepSQL  = "INSERT INTO Products (" . implode(',',$this->cols) . ") VALUES (";
$PrepSQL .= '?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)";
$stmt = $m->prepare($PrepSQL);
if(!$stmt) die('Could not prepare: ' . $m->error . "\n$PrepSQL\n");
$ret = $stmt->bind_param('isissssddssssssssssssssssssssssssisssssss',
      $contents[0], ... bunch of these here ... );
if(!$ret) die('bind_param failed: ' . $m->error);

Then in a loop I have:

$contents = explode('|',$NL); // NL is pipe delimited data
print_r($contents); 
if(!$stmt->execute()) die("Statement failed: " . $m->error);

And the script doesn't halt but every value is null in MySQL. Most of the types are strings and I would assume they would at least fill in even if the numeric types are wrong. The print_r is printing values correctly.

Thanks!