diff --git a/include/anthem/StatementVisitor.h b/include/anthem/StatementVisitor.h
index 28e38e9..74517d4 100644
--- a/include/anthem/StatementVisitor.h
+++ b/include/anthem/StatementVisitor.h
@@ -214,7 +214,7 @@ struct StatementVisitor
 		if (aritySymbol.type() != Clingo::SymbolType::Number)
 			fail();
 
-		const size_t arity = arityArgument.data.get<Clingo::Symbol>().number();
+		const size_t arity = aritySymbol.number();
 
 		if (!context.externalPredicateSignatures)
 			context.externalPredicateSignatures.emplace();