diff --git a/src/FFMpeg/Format/Video/DefaultVideo.php b/src/FFMpeg/Format/Video/DefaultVideo.php
index fcbb26f..a6d6c5f 100644
--- a/src/FFMpeg/Format/Video/DefaultVideo.php
+++ b/src/FFMpeg/Format/Video/DefaultVideo.php
@@ -97,8 +97,13 @@ abstract class DefaultVideo extends DefaultAudio implements Interactive, Resampl
                 break;
             case self::RESIZEMODE_FIT:
             default:
-                $width = $this->width;
-                $height = $this->height;
+                if (null !== $this->width && null !== $this->height) {
+                    $width = $this->width;
+                    $height = $this->height;
+                } else {
+                    $width = $originalWidth;
+                    $height = $originalHeight;
+                }
                 break;
         }