diff --git a/htdocs/api/class/api.class.php b/htdocs/api/class/api.class.php
index ab65576811a..b4e1abdbadb 100644
--- a/htdocs/api/class/api.class.php
+++ b/htdocs/api/class/api.class.php
@@ -310,14 +310,23 @@ class DolibarrApi
 			}
 			if ($tmp[$i] == ')') {
 				$counter--;
+
+				// TODO: After a closing ), only a " or " or " and " or end of string is allowed.
 			}
 			if ($counter < 0) {
-				$error = "Bad sqlfilters=".$sqlfilters;
+				$error = "Bad sqlfilters (too many closing parenthesis) = ".$sqlfilters;
 				dol_syslog($error, LOG_WARNING);
 				return false;
 			}
 			$i++;
 		}
+
+		if ($counter > 0) {
+			$error = "Bad sqlfilters (too many opening parenthesis) = ".$sqlfilters;
+			dol_syslog($error, LOG_WARNING);
+			return false;
+		}
+
 		return true;
 	}
 
@@ -327,7 +336,8 @@ class DolibarrApi
 	 * Function to forge a SQL criteria
 	 *
 	 * @param  array    $matches    Array of found string by regex search.
-	 * 								Example: "t.ref:like:'SO-%'" or "t.date_creation:<:'20160101'" or "t.date_creation:<:'2016-01-01 12:30:00'" or "t.nature:is:NULL" or "t.nature:isnot:NULL"
+	 * 								Each entry is 1 and only 1 criteria.
+	 * 								Example: "t.ref:like:'SO-%'", "t.date_creation:<:'20160101'", "t.date_creation:<:'2016-01-01 12:30:00'", "t.nature:is:NULL", "t.field2:isnot:NULL"
 	 * @return string               Forged criteria. Example: "t.field like 'abc%'"
 	 */
 	protected static function _forge_criteria_callback($matches)