diff --git a/src/libslic3r/Config.hpp b/src/libslic3r/Config.hpp
index ef3dc32c81..23be26ee71 100644
--- a/src/libslic3r/Config.hpp
+++ b/src/libslic3r/Config.hpp
@@ -776,7 +776,7 @@ public:
     static int	 			nil_value() { return std::numeric_limits<int>::max(); }
     // A scalar is nil, or all values of a vector are nil.
     bool 					is_nil() const override { for (auto v : this->values) if (v != nil_value()) return false; return true; }
-    bool 					is_nil(size_t idx) const override { return this->values[idx] == nil_value(); }
+    bool 					is_nil(size_t idx) const override { return values[idx < this->values.size() ? idx : 0] == nil_value(); }
 
     std::string serialize() const override
     {