diff --git a/apps/files_sharing/lib/api.php b/apps/files_sharing/lib/api.php
index ff56c465b29a5cce3853b47880895335a606ae39..dc4e5cf6c49f03ad1b69782f49fcd92cf4c5ddf0 100644
--- a/apps/files_sharing/lib/api.php
+++ b/apps/files_sharing/lib/api.php
@@ -58,7 +58,9 @@ class Api {
 			return new \OC_OCS_Result(null, 404, 'could not get shares');
 		} else {
 			foreach ($shares as &$share) {
-				if ($share['item_type'] === 'file') {
+				// file_target might not be set if the target user hasn't mounted
+				// the filesystem yet
+				if ($share['item_type'] === 'file' && isset($share['file_target'])) {
 					$share['mimetype'] = \OC_Helper::getFileNameMimeType($share['file_target']);
 				}
 				$newShares[] = $share;