diff --git a/expression/constant_fold.go b/expression/constant_fold.go
index 8045838d594ef..3f1913a0ce91e 100644
--- a/expression/constant_fold.go
+++ b/expression/constant_fold.go
@@ -97,7 +97,7 @@ func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
 					foldedExpr.GetType().Decimal = expr.GetType().Decimal
 					return foldedExpr, isDeferredConst
 				}
-				return BuildCastFunction(expr.GetCtx(), foldedExpr, foldedExpr.GetType()), isDeferredConst
+				return foldedExpr, isDeferredConst
 			}
 		} else {
 			// for no-const, here should return directly, because the following branches are unknown to be run or not
diff --git a/expression/integration_test.go b/expression/integration_test.go
index e80190808d0f6..80b627487db88 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -3592,6 +3592,7 @@ func (s *testIntegrationSuite) TestAggregationBuiltin(c *C) {
 	defer s.cleanEnv(c)
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
 	tk.MustExec("create table t(a decimal(7, 6))")
 	tk.MustExec("insert into t values(1.123456), (1.123456)")
 	result := tk.MustQuery("select avg(a) from t")
@@ -3618,6 +3619,19 @@ func (s *testIntegrationSuite) TestAggregationBuiltin(c *C) {
 	result.Check(testkit.Rows("18446744073709551615"))
 }
 
+func (s *testIntegrationSuite) Test19387(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("USE test;")
+
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t(a decimal(16, 2));")
+	tk.MustExec("select sum(case when 1 then a end) from t group by a;")
+	res := tk.MustQuery("show create table t")
+	c.Assert(len(res.Rows()), Equals, 1)
+	str := res.Rows()[0][1].(string)
+	c.Assert(strings.Contains(str, "decimal(16,2)"), IsTrue)
+}
+
 func (s *testIntegrationSuite) TestAggregationBuiltinBitOr(c *C) {
 	defer s.cleanEnv(c)
 	tk := testkit.NewTestKit(c, s.store)