diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index 2da271b8c1a9a..b575bb541ac4f 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -1718,6 +1718,9 @@ func (c *unhexFunctionClass) getFunction(ctx sessionctx.Context, args []Expressi
 	default:
 		return nil, errors.Errorf("Unhex invalid args, need int or string but get %s", argType)
 	}
+	if argType.GetFlen() == types.UnspecifiedLength {
+		retFlen = types.UnspecifiedLength
+	}
 
 	bf, err := newBaseBuiltinFuncWithTp(ctx, c.funcName, args, types.ETString, types.ETString)
 	if err != nil {
diff --git a/planner/core/plan_cache_test.go b/planner/core/plan_cache_test.go
index d658374a24d6b..233378bf5e5fb 100644
--- a/planner/core/plan_cache_test.go
+++ b/planner/core/plan_cache_test.go
@@ -2367,6 +2367,19 @@ func TestIssue45253(t *testing.T) {
 	tk.MustQuery(`SELECT c1 FROM t1 WHERE TO_BASE64('')`).Check(testkit.Rows())
 }
 
+func TestIssue45378(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec(`set tidb_enable_non_prepared_plan_cache=1`)
+	tk.MustExec(`CREATE TABLE t1(c1 INT)`)
+	tk.MustExec(`INSERT INTO t1 VALUES (1)`)
+
+	tk.MustQuery(`SELECT c1 FROM t1 WHERE UNHEX(2038330881)`).Check(testkit.Rows("1"))
+	tk.MustQuery(`SELECT c1 FROM t1 WHERE UNHEX(2038330881)`).Check(testkit.Rows("1"))
+	tk.MustQuery(`select @@last_plan_from_cache`).Check(testkit.Rows("1"))
+}
+
 func TestNonPreparedPlanCacheBuiltinFuncs(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)