diff --git a/lib/private/image.php b/lib/private/image.php
index 4aa14be109c53d08ac405be37e36bf157f8305c3..1cd7399cce1aa1e5c33a6bbf04e51e8b935b8963 100644
--- a/lib/private/image.php
+++ b/lib/private/image.php
@@ -217,7 +217,7 @@ class OC_Image {
 					$imageType = IMAGETYPE_BMP;
 					break;
 				default:
-					$imageType = IMAGETYPE_PNG;
+					throw new Exception('\OC_Image::_output(): "' . $mimeType . '" is not supported when forcing a specific output format');
 					break;
 			}
 		} else {