Getting error message when saving conversation as a variable

I’m using your plug-in to do a currency conversion as a hidden input field within a front-end form. The code is <input type="hidden" name="wpcf-feepounds" value="[currency amount="[types field="fee" id="$sell-list"][/types]" from="[wpv-post-author id="$sell-list" format="meta" meta="wpcf-seller-default-currency-code"]" to="GBP" show_from=false append="" round_append=""]"> It is working but I am getting a visible error message on screen, when the form is submitted, as follows; ‘Warning: number_format() expects parameter 1 to be double, string given in /home/audiogea/public_html/wp-content/plugins/euro-fxref-currency-converter/eurofxref.php on line 108’ As a test, even if I simply display a conversion within the form, e.g. [currency amount="[types field="price" id="$sell-list"][/types]" from="[wpv-post-author id="$sell-list" format="meta" meta="wpcf-seller-default-currency-code"]" to="[types usermeta="buyer-default-currency-code" user_current="true"][/types]" show_from=false append="" round_append=""] then I still get the same error message when the form is submitted. I’d be grateful for any help to get this error message to disappear. Regards Robert https://wordpress.org/plugins/euro-fxref-currency-converter/