This occurs when text has been entered into number field when filling out the form. The end user will be prompted that there is an error on the form that they will need to amend before submitting. 

The error Conversion from string "" to type 'Integer' is not valid will be displayed within the form error(s) within the Online Express plugin. This is just a recording that the error has occurred on the form, users do not need to action this further and it can be ignored.