GWT 1.7 exception in integration with existing project

I tried to put GWT in an existing project and after my first try to make a gwt compile, I got the following exception:

java.lang.NoSuchMethodError: org.eclipse.jdt.internal.compiler.lookup.ProblemReferenceBinding.closestReferenceMatch()Lorg/eclipse/jdt/internal/compiler/lookup/ReferenceBinding;

The problem exist, because the existing libraries in my project had an conflict with the gwt libraries. Putting the gwt libraries in a higher position in the build path fixes the problem.