diff --git a/src/Attr.cc b/src/Attr.cc
index 69e8fde07e..00c7c16bbb 100644
--- a/src/Attr.cc
+++ b/src/Attr.cc
@@ -43,7 +43,10 @@ const char* attr_name(AttrTag t)
 		"&ordered",
 	};
 
-	return attr_names[int(t)];
+	if ( int(t) >= 0 && int(t) < NUM_ATTRS )
+		return attr_names[int(t)];
+	else
+		return "<unknown>";
 	}
 
 Attr::Attr(AttrTag t, ExprPtr e) : expr(std::move(e))