diff --git a/install/lib/install.lib.php b/install/lib/install.lib.php
index 473dc9f45bde291d6460e56d266f2a59644acf92..b624be4e1d7b1f76abbd15c2e8ea75b1bf39497b 100644
--- a/install/lib/install.lib.php
+++ b/install/lib/install.lib.php
@@ -556,7 +556,7 @@ function removeLine($filename,$search_pattern,$strict = 0) {
 
 function is_installed($appname) {
 	exec('which '.escapeshellcmd($appname),$out);
-	if(stristr($out[0],$appname)) {
+	if(isset($out[0]) && stristr($out[0],$appname)) {
 		return true;
 	} else {
 		return false;