diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index 72c6bd5cf8..3ea1d56009 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -3531,7 +3531,12 @@ func (c *toBase64FunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	charset, collate := ctx.GetSessionVars().GetCharsetInfo()
 	bf.tp.SetCharset(charset)
 	bf.tp.SetCollate(collate)
-	bf.tp.SetFlen(base64NeededEncodedLength(bf.args[0].GetType().GetFlen()))
+
+	if bf.args[0].GetType().GetFlen() == types.UnspecifiedLength {
+		bf.tp.SetFlen(types.UnspecifiedLength)
+	} else {
+		bf.tp.SetFlen(base64NeededEncodedLength(bf.args[0].GetType().GetFlen()))
+	}
 
 	valStr, _ := ctx.GetSessionVars().GetSystemVar(variable.MaxAllowedPacket)
 	maxAllowedPacket, err := strconv.ParseUint(valStr, 10, 64)
diff --git a/planner/core/plan_cache_test.go b/planner/core/plan_cache_test.go
index 499d9ec7d3..7e061cef77 100644
--- a/planner/core/plan_cache_test.go
+++ b/planner/core/plan_cache_test.go
@@ -2417,6 +2417,22 @@ func TestIssue43667(t *testing.T) {
 	tk.MustQueryWithContext(tctx, `select (val) from cycle where pk = 4`).Check(testkit.Rows("4"))
 }
 
+func TestIssue45253(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 TO_BASE64('牵')`).Check(testkit.Rows("1"))
+	tk.MustQuery(`SELECT c1 FROM t1 WHERE TO_BASE64('牵')`).Check(testkit.Rows("1"))
+	tk.MustQuery(`select @@last_plan_from_cache`).Check(testkit.Rows("1"))
+	tk.MustQuery(`SELECT c1 FROM t1 WHERE TO_BASE64('哈')`).Check(testkit.Rows("1"))
+	tk.MustQuery(`select @@last_plan_from_cache`).Check(testkit.Rows("1"))
+	tk.MustQuery(`SELECT c1 FROM t1 WHERE TO_BASE64('')`).Check(testkit.Rows())
+}
+
 func TestNonPreparedPlanCacheBuiltinFuncs(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)