Getting fatal error trying to use custom option

I added custom option targeted to a specific product, and the option field is displayed as expected; however getting the below error after entering data in the field and attempting to add product to cart:

Fatal error: WEPOF_Product_Options_Frontend::validate_field(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "WEPOF_Product_Field_InputText" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /homepages/8/d209994786/htdocs/clickandbuilds/FaithfulCreations/wp-content/plugins/woo-extra-product-options/classes/fe/class-wepof-product-options-frontend.php on line 225

Am I doing something wrong?

Thanks,
mark