warning: explode() [function.explode]: Empty delimiter

warning: explode() [function.explode]: Empty delimiter in /xxx/sites/all/modules/invoice/invoice_helpers.inc on line 98.
warning: implode() [function.implode]: Invalid arguments passed in /xxx/sites/all/modules/invoice/invoice_helpers.inc on line 111.

This repeats about a dozen times. The line of code in question is:
$number = implode($locale['mon_decimal_point'], $number);

The problem is $locale['mon_decimal_point'] is an empty string.

decimal_point (String, 1 characters ) .
mon_decimal_point (String, 0 characters )

So I changed it to decimal_point for the time being.