diff --git a/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/AbstractLocaleResolver.java b/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/AbstractLocaleResolver.java
index 2c5674b07e85e..d5d2f70d344fb 100644
--- a/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/AbstractLocaleResolver.java
+++ b/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/AbstractLocaleResolver.java
@@ -8,12 +8,17 @@
 import org.hibernate.validator.spi.messageinterpolation.LocaleResolver;
 import org.hibernate.validator.spi.messageinterpolation.LocaleResolverContext;
 import org.jboss.logging.Logger;
+import org.jboss.resteasy.reactive.common.util.CaseInsensitiveMap;
 
 abstract class AbstractLocaleResolver implements LocaleResolver {
 
     private static final Logger log = Logger.getLogger(AbstractLocaleResolver.class);
     private static final String ACCEPT_HEADER = "Accept-Language";
 
+    /**
+     * @return case-insensitive map
+     * @see CaseInsensitiveMap
+     */
     protected abstract Map<String, List<String>> getHeaders();
 
     @Override
diff --git a/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/ResteasyReactiveLocaleResolver.java b/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/ResteasyReactiveLocaleResolver.java
index ff3565546883c..7756a2f547626 100644
--- a/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/ResteasyReactiveLocaleResolver.java
+++ b/extensions/hibernate-validator/runtime/src/main/java/io/quarkus/hibernate/validator/runtime/locale/ResteasyReactiveLocaleResolver.java
@@ -1,12 +1,13 @@
 package io.quarkus.hibernate.validator.runtime.locale;
 
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
 import jakarta.inject.Inject;
 import jakarta.inject.Singleton;
 
+import org.jboss.resteasy.reactive.common.util.CaseInsensitiveMap;
+
 import io.quarkus.arc.Arc;
 import io.quarkus.arc.ManagedContext;
 import io.quarkus.vertx.http.runtime.CurrentVertxRequest;
@@ -31,7 +32,7 @@ protected Map<String, List<String>> getHeaders() {
         }
         RoutingContext current = currentVertxRequest.getCurrent();
         if (current != null) {
-            Map<String, List<String>> result = new HashMap<>();
+            Map<String, List<String>> result = new CaseInsensitiveMap();
             MultiMap headers = current.request().headers();
             for (String name : headers.names()) {
                 result.put(name, headers.getAll(name));
diff --git a/integration-tests/hibernate-validator-resteasy-reactive/src/main/java/io/quarkus/it/hibernate/validator/TextResource.java b/integration-tests/hibernate-validator-resteasy-reactive/src/main/java/io/quarkus/it/hibernate/validator/TextResource.java
index 7813814e53031..9c5148c026cdf 100644
--- a/integration-tests/hibernate-validator-resteasy-reactive/src/main/java/io/quarkus/it/hibernate/validator/TextResource.java
+++ b/integration-tests/hibernate-validator-resteasy-reactive/src/main/java/io/quarkus/it/hibernate/validator/TextResource.java
@@ -13,7 +13,7 @@ public class TextResource {
     @GET
     @Path("/validate/{id}")
     public String validate(
-            @Digits(integer = 5, fraction = 0, message = "numeric value out of bounds") @PathParam("id") String id) {
+            @Digits(integer = 5, fraction = 0) @PathParam("id") String id) {
         return id;
     }
 
@@ -21,7 +21,7 @@ public String validate(
     @Path("/validate/text/{id}")
     @Produces(MediaType.TEXT_PLAIN)
     public String validateText(
-            @Digits(integer = 5, fraction = 0, message = "numeric value out of bounds") @PathParam("id") String id) {
+            @Digits(integer = 5, fraction = 0) @PathParam("id") String id) {
         return id;
     }
 }
diff --git a/integration-tests/hibernate-validator-resteasy-reactive/src/main/resources/application.properties b/integration-tests/hibernate-validator-resteasy-reactive/src/main/resources/application.properties
new file mode 100644
index 0000000000000..e5cfac536ba43
--- /dev/null
+++ b/integration-tests/hibernate-validator-resteasy-reactive/src/main/resources/application.properties
@@ -0,0 +1,2 @@
+quarkus.locales=en,de
+quarkus.default-locale=en
\ No newline at end of file
diff --git a/integration-tests/hibernate-validator-resteasy-reactive/src/test/java/io/quarkus/it/hibernate/validator/TextResourceTest.java b/integration-tests/hibernate-validator-resteasy-reactive/src/test/java/io/quarkus/it/hibernate/validator/TextResourceTest.java
index b7fc426b18367..3638992514223 100644
--- a/integration-tests/hibernate-validator-resteasy-reactive/src/test/java/io/quarkus/it/hibernate/validator/TextResourceTest.java
+++ b/integration-tests/hibernate-validator-resteasy-reactive/src/test/java/io/quarkus/it/hibernate/validator/TextResourceTest.java
@@ -1,6 +1,8 @@
 package io.quarkus.it.hibernate.validator;
 
+import static io.restassured.RestAssured.given;
 import static io.restassured.RestAssured.when;
+import static org.hamcrest.Matchers.containsString;
 
 import org.junit.jupiter.api.Test;
 
@@ -25,4 +27,22 @@ public void fetchText() {
                 .statusCode(400)
                 .contentType(ContentType.TEXT);
     }
+
+    @Test
+    public void shouldGetAcceptLanguageLocaleIfKeyIsUpperCase() {
+        given()
+                .header("Accept-Language", "de")
+                .when().get("/text/validate/boom")
+                .then().log().ifValidationFails()
+                .body(containsString("numerischer Wert außerhalb des gültigen Bereichs"));
+    }
+
+    @Test
+    public void shouldGetAcceptLanguageLocaleIfKeyIsLowerCase() {
+        given()
+                .header("accept-language", "de")
+                .when().get("/text/validate/boom")
+                .then().log().ifValidationFails()
+                .body(containsString("numerischer Wert außerhalb des gültigen Bereichs"));
+    }
 }