diff --git a/.github/workflows/build-tzpb.yml b/.github/workflows/build-tzpb.yml
index db36350..bf922ab 100644
--- a/.github/workflows/build-tzpb.yml
+++ b/.github/workflows/build-tzpb.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.21"]
+        go: ["1.20", "1.21"]
 
     steps:
       - uses: actions/checkout@v3
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2808fbd..bddd0cf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -28,7 +28,7 @@ jobs:
     name: golangci-lint
     strategy:
       matrix:
-        go: ["1.21"]
+        go: ["1.20", "1.21"]
     permissions:
       contents: read
       pull-requests: read
@@ -50,7 +50,7 @@ jobs:
   build:
     strategy:
       matrix:
-        go: ["1.21"]
+        go: ["1.20", "1.21"]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
diff --git a/sort.go b/sort.go
new file mode 100644
index 0000000..87fba39
--- /dev/null
+++ b/sort.go
@@ -0,0 +1,12 @@
+//go:build go1.21
+
+package tzf
+
+import (
+	"cmp"
+	"slices"
+)
+
+func sort[S ~[]E, E cmp.Ordered](x S) {
+	slices.Sort(x)
+}
diff --git a/sort_before_1_21.go b/sort_before_1_21.go
new file mode 100644
index 0000000..23d51e4
--- /dev/null
+++ b/sort_before_1_21.go
@@ -0,0 +1,11 @@
+//go:build !go1.21
+
+package tzf
+
+import (
+	"golang.org/x/exp/slices"
+)
+
+func sort(x S) {
+	slices.Sort(x)
+}
diff --git a/tzf.go b/tzf.go
index 16b4ab6..5fc12e6 100644
--- a/tzf.go
+++ b/tzf.go
@@ -7,7 +7,6 @@ package tzf
 import (
 	"errors"
 	"fmt"
-	"slices"
 
 	"github.com/ringsaturn/tzf/convert"
 	"github.com/ringsaturn/tzf/pb"
@@ -241,7 +240,7 @@ func (f *Finder) GetTimezoneNames(lng float64, lat float64) ([]string, error) {
 	for i := 0; i < len(item); i++ {
 		ret = append(ret, item[i].name)
 	}
-	slices.Sort[[]string](ret)
+	sort(ret)
 	return ret, nil
 }
 
diff --git a/tzf_fuzzy.go b/tzf_fuzzy.go
index e7ccd22..78e0c9c 100644
--- a/tzf_fuzzy.go
+++ b/tzf_fuzzy.go
@@ -1,8 +1,6 @@
 package tzf
 
 import (
-	"slices"
-
 	"github.com/paulmach/orb"
 	"github.com/paulmach/orb/maptile"
 	"github.com/ringsaturn/tzf/pb"
@@ -37,7 +35,7 @@ func NewFuzzyFinderFromPB(input *pb.PreindexTimezones) (F, error) {
 		namesMap[item.Name] = true
 	}
 	f.names = maps.Keys(namesMap)
-	slices.Sort(f.names)
+	sort(f.names)
 	return f, nil
 }