diff --git a/gramjs/Utils.ts b/gramjs/Utils.ts
index ad72fcef..1e6b3bd3 100644
--- a/gramjs/Utils.ts
+++ b/gramjs/Utils.ts
@@ -236,7 +236,10 @@ export function _photoSizeByteCount(size: Api.TypePhotoSize) {
         return size.bytes.length;
     } else if (size instanceof Api.PhotoSizeEmpty) {
         return 0;
-    } else {
+    } else if (size instanceof Api.PhotoSizeProgressive) {
+        return size.sizes[size.sizes.length -1];
+    }
+    else {
         return undefined;
     }
 }
diff --git a/gramjs/tl/custom/file.ts b/gramjs/tl/custom/file.ts
index c51a6955..d8790fb3 100644
--- a/gramjs/tl/custom/file.ts
+++ b/gramjs/tl/custom/file.ts
@@ -69,7 +69,7 @@ export class File {
 
     get size() {
         if (this.media instanceof Api.Photo) {
-            return _photoSizeByteCount(this.media.sizes[-1]);
+            return _photoSizeByteCount(this.media.sizes[this.media.sizes.length -1]);
         } else if (this.media instanceof Api.Document) {
             return this.media.size;
         }