diff --git a/install/lib/installer_base.lib.php b/install/lib/installer_base.lib.php
index 95c6cb87ef00dbe54f7cacae990e1b995ef13c3d..7103c281925691f04eac14802cab1e0ead902ba0 100644
--- a/install/lib/installer_base.lib.php
+++ b/install/lib/installer_base.lib.php
@@ -1793,9 +1793,9 @@ class installer_base {
 		}
 
 		if(is_user('_rspamd') && is_group('amavis')) {
-			exec("usermod -G amavis _rspamd");
+			exec("usermod -a -G amavis _rspamd");
 		} elseif(is_user('rspamd') && is_group('amavis')) {
-			exec("usermod -G amavis rspamd");
+			exec("usermod -a -G amavis rspamd");
 		}
 
 		if(!is_dir('/etc/rspamd/local.d/')){