diff --git a/lib/ftp.php b/lib/ftp.php
index 388e132..b94d7fe 100644
--- a/lib/ftp.php
+++ b/lib/ftp.php
@@ -18,7 +18,6 @@ class OC_User_FTP extends OC_User_Backend{
 		if($this->secure) {
 			$this->protocol.='s';
 		}
-		$this->protocol.='://';
 	}
 
 	/**
@@ -30,7 +29,11 @@ class OC_User_FTP extends OC_User_Backend{
 	 * Check if the password is correct without logging in the user
 	 */
 	public function checkPassword($uid, $password) {
-		$url=$this->protocol.$uid.':'.$password.'@'.$this->host.'/';
+		if (false === array_search($this->protocol, stream_get_wrappers())) {
+			OCP\Util::writeLog('user_external', 'ERROR: Stream wrapper not available: ' . $this->protocol, OCP\Util::ERROR);
+			return false;
+		}
+		$url = sprintf('%s://%s:%s@%s/', $this->protocol, $uid, $password, $this->host);
 		$result=@opendir($url);
 		if(is_resource($result)) {
 			return $uid;
diff --git a/lib/imap.php b/lib/imap.php
index 201d9d8..d1c6263 100644
--- a/lib/imap.php
+++ b/lib/imap.php
@@ -22,6 +22,10 @@ class OC_User_IMAP extends OC_User_Backend{
 	 * Check if the password is correct without logging in the user
 	 */
 	public function checkPassword($uid, $password) {
+		if (!function_exists('imap_open')) {
+			OCP\Util::writeLog('user_external', 'ERROR: PHP imap extension is not installed', OCP\Util::ERROR);
+			return false;
+		}
 		$mbox = @imap_open($this->mailbox, $uid, $password);
 		imap_errors();
 		imap_alerts();
diff --git a/lib/smb.php b/lib/smb.php
index 96429d8..8a95120 100644
--- a/lib/smb.php
+++ b/lib/smb.php
@@ -29,10 +29,18 @@ class OC_User_SMB extends OC_User_Backend{
 		$password=escapeshellarg($password);
 		$result=array();
 		$command=self::smbclient.' //'.$this->host.'/dummy -U'.$uidEscaped.'%'.$password;
-		$result=exec($command,$result);
-		if(substr($result,-strlen(self::loginError))==self::loginError) {
+		$lastline = exec($command, $output, $retval);
+		if ($retval === 127) {
+			OCP\Util::writeLog('user_external', 'ERROR: smbclient executable missing', OCP\Util::ERROR);
 			return false;
-		}else{
+		} else if (strpos($lastline, self::loginError) !== false) {
+			//normal login error
+			return false;
+		} else if ($retval != 0) {
+			//some other error
+			OCP\Util::writeLog('user_external', 'ERROR: smbclient error: ' . trim($lastline), OCP\Util::ERROR);
+			return false;
+		} else {
 			return $uid;
 		}
 	}