diff --git a/expression/collation.go b/expression/collation.go
index c449f1fb7a6a0..29821094efca4 100644
--- a/expression/collation.go
+++ b/expression/collation.go
@@ -140,7 +140,7 @@ func deriveCoercibilityForScarlarFunc(sf *ScalarFunction) Coercibility {
 	if _, ok := sysConstFuncs[sf.FuncName.L]; ok {
 		return CoercibilitySysconst
 	}
-	if !types.IsString(sf.RetType.Tp) {
+	if sf.RetType.EvalType() != types.ETString {
 		return CoercibilityNumeric
 	}
 	coer := CoercibilityIgnorable
@@ -155,14 +155,14 @@ func deriveCoercibilityForScarlarFunc(sf *ScalarFunction) Coercibility {
 func deriveCoercibilityForConstant(c *Constant) Coercibility {
 	if c.Value.IsNull() {
 		return CoercibilityIgnorable
-	} else if !types.IsString(c.RetType.Tp) {
+	} else if c.RetType.EvalType() != types.ETString {
 		return CoercibilityNumeric
 	}
 	return CoercibilityCoercible
 }
 
 func deriveCoercibilityForColumn(c *Column) Coercibility {
-	if !types.IsString(c.RetType.Tp) {
+	if c.RetType.EvalType() != types.ETString {
 		return CoercibilityNumeric
 	}
 	return CoercibilityImplicit
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 9ae1d20ccf70b..18679b342a231 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -6685,6 +6685,18 @@ func (s *testIntegrationSuite) TestIssue17727(c *C) {
 	tk.MustQuery("select @@last_plan_from_cache;").Check(testkit.Rows("1"))
 }
 
+func (s *testIntegrationSerialSuite) TestIssue20268(c *C) {
+	collate.SetNewCollationEnabledForTest(true)
+	defer collate.SetNewCollationEnabledForTest(false)
+
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("CREATE TABLE `t` (   `a` enum('a','b') DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;")
+	tk.MustExec("insert into t values('a');")
+	tk.MustExec("select * from t where a = 'A';")
+}
+
 func (s *testIntegrationSuite) TestIssue18515(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")