diff --git a/inc/Service/Media/DefaultImageFormatReader.php b/inc/Service/Media/DefaultImageFormatReader.php
index 7487ecdc..f40a0b7d 100644
--- a/inc/Service/Media/DefaultImageFormatReader.php
+++ b/inc/Service/Media/DefaultImageFormatReader.php
@@ -2,17 +2,25 @@
 namespace Vichan\Service\Media;
 
 
+/**
+ * Do not use this if you can.
+ *
+ * Some formats may contain no image or may contain multiple images. In these cases, getimagesize() might not be
+ * able to properly determine the image size. getimagesize() will return zero for width and height in these cases.
+ *
+ * getimagesize() is agnostic of any image metadata.
+ * If e.g. the Exif Orientation flag is set to a value which rotates the image by 90 or 270 degress, index 0 and 1
+ * are swapped, i.e. the contain the height and width, respectively.
+ */
 class DefaultImageFormatReader implements ImageFormatReader {
-	/**
-	 * getimagesize() is agnostic of any image metadata.
-	 * If e.g. the Exif Orientation flag is set to a value which rotates the image by 90 or 270 degress, index 0 and 1
-	 * are swapped, i.e. the contain the height and width, respectively.
-	 */
 	public function getSizes(string $file_path): array {
-		$ret = \getimagesize($file_path);
+		$ret = \getimagesize($file_path, $info);
 		if ($ret === false) {
 			throw new \RuntimeException("Could not read image sizes of '$file_path'");
 		}
+		if ($ret[2] == \IMAGETYPE_UNKNOWN) {
+			throw new \RuntimeException("Error '$file_path' is not an image");
+		}
 		return $ret;
 	}
 }