diff --git a/lib/pddlparse/include/pddlparse/detail/VariableStack.h b/lib/pddlparse/include/pddlparse/detail/VariableStack.h
index 7482f6d..9153f15 100644
--- a/lib/pddlparse/include/pddlparse/detail/VariableStack.h
+++ b/lib/pddlparse/include/pddlparse/detail/VariableStack.h
@@ -23,7 +23,7 @@ class VariableStack
 		void push(Layer layer);
 		void pop();
 
-		std::experimental::optional<ast::VariableDeclaration *> findVariableDeclaration(const std::string &variableName);
+		std::experimental::optional<ast::VariableDeclaration *> findVariableDeclaration(const std::string &variableName) const;
 		bool contains(const ast::VariableDeclaration &variableDeclaration) const;
 
 	private:
diff --git a/lib/pddlparse/src/pddlparse/detail/VariableStack.cpp b/lib/pddlparse/src/pddlparse/detail/VariableStack.cpp
index 7be8ab9..3f53555 100644
--- a/lib/pddlparse/src/pddlparse/detail/VariableStack.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/VariableStack.cpp
@@ -29,7 +29,7 @@ void VariableStack::pop()
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-std::experimental::optional<ast::VariableDeclaration *> VariableStack::findVariableDeclaration(const std::string &variableName)
+std::experimental::optional<ast::VariableDeclaration *> VariableStack::findVariableDeclaration(const std::string &variableName) const
 {
 	const auto variableDeclarationMatches =
 		[&variableName](const auto &variableDeclaration)