diff --git a/phpgwapi/inc/class.egw_db.inc.php b/phpgwapi/inc/class.egw_db.inc.php
index 733835dd7c..8fea7e2f00 100644
--- a/phpgwapi/inc/class.egw_db.inc.php
+++ b/phpgwapi/inc/class.egw_db.inc.php
@@ -1474,6 +1474,10 @@ class egw_db
 		{
 			$value = substr($value,0,$length);
 		}
+		// casting boolean explicitly to string, as ADODB_postgres64::qstr() has an unwanted special handling
+		// for boolean types, causing it to return "true" or "false" and not a quoted string like "'1'"!
+		if (is_bool($value)) $value = (string)$value;
+
 		return $this->Link_ID->qstr($value);
 	}