Annotation processor fails in Eclipse when evaluating custom constraints not defined in the current project

In certain situations the AP fails when being integrated in Eclipse with the following exception:

java.lang.AssertionError: Class org.hibernate.validator.constraints.impl.LengthValidator specified in @Constraint.validatedBy doesn't implement ConstraintValidator.
at org.hibernate.validator.ap.util.ConstraintHelper.getConstraintValidatorSuperType(ConstraintHelper.java:497)
at org.hibernate.validator.ap.util.ConstraintHelper.getSupportedType(ConstraintHelper.java:426)
at org.hibernate.validator.ap.util.ConstraintHelper.getSupportedTypes(ConstraintHelper.java:402)
at org.hibernate.validator.ap.util.ConstraintHelper.checkCustomConstraint(ConstraintHelper.java:356)
at org.hibernate.validator.ap.util.ConstraintHelper.checkConstraint(ConstraintHelper.java:295)
at org.hibernate.validator.ap.ConstraintAnnotationVisitor.checkConstraintAtField(ConstraintAnnotationVisitor.java:290)
at org.hibernate.validator.ap.ConstraintAnnotationVisitor.visitVariableAsField(ConstraintAnnotationVisitor.java:174)
at org.hibernate.validator.ap.ConstraintAnnotationVisitor.visitVariableAsField(ConstraintAnnotationVisitor.java:45)
at javax.lang.model.util.ElementKindVisitor6.visitVariable(ElementKindVisitor6.java:199)
at org.eclipse.jdt.internal.compiler.apt.model.VariableElementImpl.accept(VariableElementImpl.java:55)
at org.hibernate.validator.ap.ConstraintValidationProcessor.process(ConstraintValidationProcessor.java:84)
at org.eclipse.jdt.internal.compiler.apt.dispatch.RoundDispatcher.handleProcessor(RoundDispatcher.java:139)
at org.eclipse.jdt.internal.compiler.apt.dispatch.RoundDispatcher.round(RoundDispatcher.java:121)
at org.eclipse.jdt.internal.compiler.apt.dispatch.BaseAnnotationProcessorManager.processAnnotations(BaseAnnotationProcessorManager.java:159)
at org.eclipse.jdt.internal.apt.pluggable.core.dispatch.IdeAnnotationProcessorManager.processAnnotations(IdeAnnotationProcessorManager.java:134)
at org.eclipse.jdt.internal.compiler.Compiler.processAnnotations(Compiler.java:810)
at org.eclipse.jdt.internal.compiler.Compiler.compile(Compiler.java:428)
at org.eclipse.jdt.internal.core.builder.AbstractImageBuilder.compile(AbstractImageBuilder.java:364)
at org.eclipse.jdt.internal.core.builder.IncrementalImageBuilder.compile(IncrementalImageBuilder.java:321)
at org.eclipse.jdt.internal.core.builder.AbstractImageBuilder.compile(AbstractImageBuilder.java:301)
at org.eclipse.jdt.internal.core.builder.IncrementalImageBuilder.build(IncrementalImageBuilder.java:134)
at org.eclipse.jdt.internal.core.builder.JavaBuilder.buildDeltas(JavaBuilder.java:265)
at org.eclipse.jdt.internal.core.builder.JavaBuilder.build(JavaBuilder.java:193)
at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:627)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:170)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:201)
at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:253)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:256)
at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:309)
at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:341)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:140)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:238)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)

This happens, if the AP examines a constraint annotation, which is not defined in the currently built project itself, but is imported from some other JAR (e.g. @Length from HV):

public class TestModel {

@Length
private String test;
}

The problem occurs, regardless whether the JAR containing the constraint is on the AP classpath or not.

The AP works as expected when running on javac.