diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index c15e4d4200920..9ac2eb370d380 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -289,7 +289,7 @@ func (c *concatFunctionClass) getFunction(ctx sessionctx.Context, args []Express
 
 		if argType.Flen < 0 {
 			bf.tp.Flen = mysql.MaxBlobWidth
-			logutil.BgLogger().Warn("unexpected `Flen` value(-1) in CONCAT's args", zap.Int("arg's index", i))
+			logutil.BgLogger().Debug("unexpected `Flen` value(-1) in CONCAT's args", zap.Int("arg's index", i))
 		}
 		bf.tp.Flen += argType.Flen
 	}
@@ -365,7 +365,7 @@ func (c *concatWSFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 		if i != 0 {
 			if argType.Flen < 0 {
 				bf.tp.Flen = mysql.MaxBlobWidth
-				logutil.BgLogger().Warn("unexpected `Flen` value(-1) in CONCAT_WS's args", zap.Int("arg's index", i))
+				logutil.BgLogger().Debug("unexpected `Flen` value(-1) in CONCAT_WS's args", zap.Int("arg's index", i))
 			}
 			bf.tp.Flen += argType.Flen
 		}