diff --git a/loader.go b/loader.go
index 53c51a7..3dbe1fc 100644
--- a/loader.go
+++ b/loader.go
@@ -1,6 +1,7 @@
 package i18n
 
 import (
+	"errors"
 	"io/fs"
 	"path"
 	"slices"
@@ -30,14 +31,21 @@ func (l *FSLoader) Load(lang string) (map[string]Message, *GrammarData, error) {
 	variants := localeFilenameCandidates(lang)
 	var data []byte
 	var err error
+	var firstNonMissingErr error
 	for _, filename := range variants {
 		filePath := path.Join(l.dir, filename)
 		data, err = fs.ReadFile(l.fsys, filePath)
 		if err == nil {
 			break
 		}
+		if firstNonMissingErr == nil && !errors.Is(err, fs.ErrNotExist) {
+			firstNonMissingErr = err
+		}
 	}
 	if err != nil {
+		if firstNonMissingErr != nil {
+			err = firstNonMissingErr
+		}
 		return nil, nil, log.E("FSLoader.Load", "locale not found: "+lang, err)
 	}
 
@@ -70,11 +78,21 @@ func localeFilenameCandidates(lang string) []string {
 		}
 		variants = append(variants, candidate)
 	}
-	addVariant(lang + ".json")
-	addVariant(core.Replace(lang, "-", "_") + ".json")
-	addVariant(core.Replace(lang, "_", "-") + ".json")
-	if base := baseLanguageTag(lang); base != "" && base != lang {
-		addVariant(base + ".json")
+	canonical := normalizeLanguageTag(lang)
+	addTag := func(tag string) {
+		if tag == "" {
+			return
+		}
+		addVariant(tag + ".json")
+		addVariant(core.Replace(tag, "-", "_") + ".json")
+		addVariant(core.Replace(tag, "_", "-") + ".json")
+	}
+	addTag(lang)
+	if canonical != "" && canonical != lang {
+		addTag(canonical)
+	}
+	if base := baseLanguageTag(canonical); base != "" && base != canonical {
+		addTag(base)
 	}
 	return variants
 }
diff --git a/loader_test.go b/loader_test.go
index 2a16f4f..c5d0ada 100644
--- a/loader_test.go
+++ b/loader_test.go
@@ -187,6 +187,33 @@ func TestLocaleFilenameCandidates(t *testing.T) {
 	}
 }
 
+func TestLocaleFilenameCandidatesNormalisesCase(t *testing.T) {
+	got := localeFilenameCandidates("en-us")
+	want := []string{"en-us.json", "en_us.json", "en-US.json", "en_US.json", "en.json"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("localeFilenameCandidates(en-us) = %v, want %v", got, want)
+	}
+}
+
+func TestFSLoaderLoadUsesCanonicalVariant(t *testing.T) {
+	fs := fstest.MapFS{
+		"locales/en-US.json": &fstest.MapFile{
+			Data: []byte(`{
+				"greeting": "hello"
+			}`),
+		},
+	}
+
+	loader := NewFSLoader(fs, "locales")
+	messages, _, err := loader.Load("en-us")
+	if err != nil {
+		t.Fatalf("Load(en-us) error: %v", err)
+	}
+	if got := messages["greeting"].Text; got != "hello" {
+		t.Fatalf("Load(en-us) greeting = %q, want %q", got, "hello")
+	}
+}
+
 func TestFlattenWithGrammar(t *testing.T) {
 	messages := make(map[string]Message)
 	grammar := &GrammarData{