find images resolution

Hi Forum,

I have tried to get alert for the links if it has 250 or 300 resolution. But if the pdf is placed, the script is replying error message.

"object does not support the property or method actualPPi"

How can i omit the check for PDF and continue to find other images resolution and alert.

var doc = app.activeDocument;

var graphics = doc.allGraphics;

var image, horRes, verRes;

for (var i = 0; i < graphics.length; i++) {

     image = graphics[i];

     if(image.actualPpi[0] ==300 ||

     image.actualPpi[0] ==250){

alert ("images are in 300 & 250 dpi");                 

          }

     }

thanks in advance forum...