diff --git a/pkg/expression/builtin_cast.go b/pkg/expression/builtin_cast.go
index 6c6324af3b37d..45e3589f86c17 100644
--- a/pkg/expression/builtin_cast.go
+++ b/pkg/expression/builtin_cast.go
@@ -455,7 +455,7 @@ func (c *castAsArrayFunctionClass) getFunction(ctx BuildContext, args []Expressi
 		return nil, ErrNotSupportedYet.GenWithStackByArgs("specifying charset for multi-valued index")
 	}
 	if arrayType.EvalType() == types.ETString && arrayType.GetFlen() == types.UnspecifiedLength {
-		return nil, ErrNotSupportedYet.GenWithStackByArgs("CAST-ing data to array of char/binary BLOBs")
+		return nil, ErrNotSupportedYet.GenWithStackByArgs("CAST-ing data to array of char/binary BLOBs with unspecified length")
 	}
 
 	bf, err := newBaseBuiltinFunc(ctx, c.funcName, args, c.tp)
diff --git a/tests/integrationtest/r/expression/multi_valued_index.result b/tests/integrationtest/r/expression/multi_valued_index.result
index 996b530da5a6e..52d40e6c115e4 100644
--- a/tests/integrationtest/r/expression/multi_valued_index.result
+++ b/tests/integrationtest/r/expression/multi_valued_index.result
@@ -28,9 +28,9 @@ Error 1235 (42000): This version of TiDB doesn't yet support 'Use of CAST( .. AS
 create table t(j json, gc json as (cast(j->'$[*]' as unsigned array)));
 Error 1235 (42000): This version of TiDB doesn't yet support 'Use of CAST( .. AS .. ARRAY) outside of functional index in CREATE(non-SELECT)/ALTER TABLE or in general expressions'
 create table t1(j json, key i1((cast(j->"$" as char array))));
-Error 1235 (42000): This version of TiDB doesn't yet support 'CAST-ing data to array of char/binary BLOBs'
+Error 1235 (42000): This version of TiDB doesn't yet support 'CAST-ing data to array of char/binary BLOBs with unspecified length'
 create table t1(j json, key i1((cast(j->"$" as binary array))));
-Error 1235 (42000): This version of TiDB doesn't yet support 'CAST-ing data to array of char/binary BLOBs'
+Error 1235 (42000): This version of TiDB doesn't yet support 'CAST-ing data to array of char/binary BLOBs with unspecified length'
 create table t1(j json, key i1((cast(j->"$" as float array))));
 Error 1235 (42000): This version of TiDB doesn't yet support 'CAST-ing data to array of float BINARY'
 create table t1(j json, key i1((cast(j->"$" as decimal(4,2) array))));