diff --git a/cmd/oidcgate/config.go b/cmd/oidcgate/config.go
index 32f76d8..83935aa 100644
--- a/cmd/oidcgate/config.go
+++ b/cmd/oidcgate/config.go
@@ -4,8 +4,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
-	"strconv"
 	"strings"
+	"unicode"
 
 	"github.com/lukaszraczylo/traefikoidc"
 	"gopkg.in/yaml.v3"
@@ -78,6 +78,10 @@ func Load(path string) (*Config, error) {
 	applyEnvOverrides(cfg)
 	applyDefaults(cfg)
 
+	if cfg.Listen == "" {
+		return nil, fmt.Errorf("config: missing required 'listen' (or OIDCGATE_LISTEN env var)")
+	}
+
 	// Force standalone semantics: trust X-Forwarded-Uri.
 	cfg.OIDC.TrustForwardedURI = true
 	return cfg, nil
@@ -152,8 +156,6 @@ func setScalarField(cfg *Config, field, value string) {
 	case "CACertPEM":
 		cfg.OIDC.CACertPEM = value
 	}
-	// strconv reserved for future int/bool fields when added to the allow-list.
-	_ = strconv.Atoi
 }
 
 func applyDefaults(cfg *Config) {
@@ -170,22 +172,20 @@ func applyDefaults(cfg *Config) {
 // Multi-letter acronyms keep their grouping: "OIDCEndSessionURL" →
 // "OIDC_END_SESSION_URL", "CACertPEM" → "CA_CERT_PEM".
 func camelToSnakeUpper(s string) string {
+	runes := []rune(s)
 	var b strings.Builder
-	for i, r := range s {
+	for i, r := range runes {
 		if i > 0 && isUpper(r) {
-			prev := rune(s[i-1])
+			prev := runes[i-1]
 			next := rune(0)
-			if i+1 < len(s) {
-				next = rune(s[i+1])
+			if i+1 < len(runes) {
+				next = runes[i+1]
 			}
 			if !isUpper(prev) || (next != 0 && !isUpper(next)) {
 				b.WriteByte('_')
 			}
 		}
-		if r >= 'a' && r <= 'z' {
-			r -= 32
-		}
-		b.WriteRune(r)
+		b.WriteRune(unicode.ToUpper(r))
 	}
 	return b.String()
 }
diff --git a/cmd/oidcgate/config_test.go b/cmd/oidcgate/config_test.go
index 5c5ecf7..aa976a5 100644
--- a/cmd/oidcgate/config_test.go
+++ b/cmd/oidcgate/config_test.go
@@ -106,3 +106,32 @@ func TestLoad_MissingFile(t *testing.T) {
 		t.Fatal("expected error for missing file")
 	}
 }
+
+func TestLoad_NestedStructRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	if err := os.WriteFile(path, []byte(`
+listen: ":8080"
+providerURL: "https://idp.example"
+clientID: "abc"
+clientSecret: "secret"
+sessionEncryptionKey: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+callbackURL: "/oauth2/callback"
+logoutURL: "/oauth2/logout"
+redis:
+  address: "redis:6379"
+  password: "redispw"
+`), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.OIDC.Redis == nil {
+		t.Fatal("redis block should populate cfg.OIDC.Redis")
+	}
+	if cfg.OIDC.Redis.Address != "redis:6379" {
+		t.Errorf("redis address: want redis:6379, got %q", cfg.OIDC.Redis.Address)
+	}
+}