diff --git a/lib/pddlparse/include/pddlparse/NormalizedASTForward.h b/lib/pddlparse/include/pddlparse/NormalizedASTForward.h
index eaacac6..6d7a8e5 100644
--- a/lib/pddlparse/include/pddlparse/NormalizedASTForward.h
+++ b/lib/pddlparse/include/pddlparse/NormalizedASTForward.h
@@ -82,6 +82,7 @@ struct Description;
 using DescriptionPointer = std::unique_ptr<Description>;
 struct Domain;
 using DomainPointer = std::unique_ptr<Domain>;
+struct InitialState;
 struct Problem;
 using ProblemPointer = std::unique_ptr<Problem>;
 enum class Requirement;