Google maps api : infowindow background transparent -

my website use google maps , display popup (infowindow) when clicking on markers. don't know why , since when, popup background transparent.

can me understand?

update after answer of dr.molle

wrong code :

var infowindow = new google.maps.infowindow({maxwidth: '500px'}), mapsmarker, i; 

good code :

var infowindow = new google.maps.infowindow({maxwidth: '500'}), mapsmarker, i; 

the source of issue seems maxwidth - setting of infowindow. api expects number, provide string 500px. set maxwidth 500