Play! Framework error in custom Constraints.Required validation message

Question

please note: play framework newbie

Intro:

I require custom validation messages, constraints.require Having done some research, I found that using the default:

@Constraints.Required

will show a message error.required which is usually found in conf/messages

See this helpful SO post for more info.

Problem:

When doing this, I receive this error on the webpage:

[CompletionException: java.lang.IllegalStateException: Error(s) binding form: {"password":["This field is required"],"email":["This field is required"]}]

and this in the console output:

[error] application - 

! @7539mkaon - Internal server error, for (POST) [/User/Login] ->

play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[[CompletionException: java.lang.IllegalStateException: Error(s) binding form: {"password":["This field is required"],"email":["This field is required"]}]]
    at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:255)
    at play.api.http.DefaultHttpErrorHandler.onServerError(HttpErrorHandler.scala:182)
    at play.core.server.AkkaHttpServer$$anonfun$$nestedInanonfun$executeHandler$1$1.applyOrElse(AkkaHttpServer.scala:252)
    at play.core.server.AkkaHttpServer$$anonfun$$nestedInanonfun$executeHandler$1$1.applyOrElse(AkkaHttpServer.scala:251)
    at scala.concurrent.Future.$anonfun$recoverWith$1(Future.scala:412)
    at scala.concurrent.impl.Promise.$anonfun$transformWith$1(Promise.scala:37)
    at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:60)
    at play.api.libs.streams.Execution$trampoline$.executeScheduled(Execution.scala:109)
    at play.api.libs.streams.Execution$trampoline$.execute(Execution.scala:71)
    at scala.concurrent.impl.CallbackRunnable.executeWithValue(Promise.scala:68)
Caused by: java.util.concurrent.CompletionException: java.lang.IllegalStateException: Error(s) binding form: {"password":["This field is required"],"email":["This field is required"]}
    at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:292)
    at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:308)
    at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:593)
    at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577)
    at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474)
    at java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:1977)
    at scala.concurrent.java8.FuturesConvertersImpl$CF.apply(FutureConvertersImpl.scala:21)
    at scala.concurrent.java8.FuturesConvertersImpl$CF.apply(FutureConvertersImpl.scala:18)
    at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:60)
    at scala.concurrent.BatchingExecutor$Batch.processBatch$1(BatchingExecutor.scala:63)
Caused by: java.lang.IllegalStateException: Error(s) binding form: {"password":["This field is required"],"email":["This field is required"]}
    at play.data.Form.get(Form.java:761)
    at controllers.User.UserController.doLogin(UserController.java:85)
    at router.Routes$$anonfun$routes$1.$anonfun$applyOrElse$10(Routes.scala:522)
    at play.core.routing.HandlerInvokerFactory$$anon$3.resultCall(HandlerInvoker.scala:136)
    at play.core.routing.HandlerInvokerFactory$$anon$3.resultCall(HandlerInvoker.scala:135)
    at play.core.routing.HandlerInvokerFactory$JavaActionInvokerFactory$$anon$8$$anon$2$$anon$1.invocation(HandlerInvoker.scala:110)
    at play.core.j.JavaAction$$anon$1.call(JavaAction.scala:78)
    at play.http.DefaultActionCreator$1.call(DefaultActionCreator.java:31)
    at play.core.j.JavaAction.$anonfun$apply$2(JavaAction.scala:113)
    at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:653)

Model UserLogin

import play.data.validation.Constraints;
import javax.validation.constraints.Pattern;

public class UserLogin {

    @Constraints.Required
    @Pattern(regexp = ".+[@].+[.].+", message = "Please check your email address")
    private String email;

    @Constraints.Required
    @Constraints.MinLength(message = "Minimum length is 8", value = 8)
    @Pattern(regexp = "[\\d]{8,}]", message = "Please check your email address")
    private String password;

    @Constraints.Required
    private Boolean bRememberMe;

    public UserLogin(){}

    public UserLogin(String email, String password, @Constraints.Required Boolean bRememberMe) {
        this.email = email;
        this.password = password;
        this.bRememberMe = bRememberMe;
    }

    //...
}

my conf/messages contains

error.required=This field is required

Question:

When commenting out the @Constraints.Required, this error disappears, What causes this error and how can I resolve it?