diff --git a/src/plasp/pddl/expressions/PrimitiveType.cpp b/src/plasp/pddl/expressions/PrimitiveType.cpp
index 97e3aa8..b6845e7 100644
--- a/src/plasp/pddl/expressions/PrimitiveType.cpp
+++ b/src/plasp/pddl/expressions/PrimitiveType.cpp
@@ -121,8 +121,11 @@ PrimitiveType *PrimitiveType::parseAndFind(Context &context, Domain &domain)
 	if (match == types.cend())
 	{
 		// Only "object" is allowed as an implicit type
-		if (typeName == "object")
+		if (typeName == "object" || typeName == "objects")
+		{
+			context.logger.parserWarning(context.parser, "Primitive type \"" + typeName + "\" should be declared");
 			types.emplace_back(std::make_unique<expressions::PrimitiveType>(typeName));
+		}
 		else
 			throw utils::ParserException(context.parser, "Type \"" + typeName + "\" used but never declared");