diff --git a/src/libslic3r/MutablePriorityQueue.hpp b/src/libslic3r/MutablePriorityQueue.hpp
index b45b8cfff5..fd3e7ac2db 100644
--- a/src/libslic3r/MutablePriorityQueue.hpp
+++ b/src/libslic3r/MutablePriorityQueue.hpp
@@ -7,6 +7,10 @@
 #include <limits>
 #include <cstdlib> // adds size_t (without std::)
 
+namespace Slic3r {
+
+constexpr auto InvalidQueueID = std::numeric_limits<size_t>::max();
+
 template<typename T, typename IndexSetter, typename LessPredicate, const bool ResetIndexWhenRemoved = false>
 class MutablePriorityQueue
 {
@@ -41,7 +45,7 @@ public:
 	bool		empty() const						{ return m_heap.empty(); }
 	T&			operator[](std::size_t idx) noexcept { return m_heap[idx]; }
 	const T&	operator[](std::size_t idx) const noexcept { return m_heap[idx]; }
-	static constexpr size_t invalid_id() { return std::numeric_limits<size_t>::max(); }
+    static constexpr size_t invalid_id() { return InvalidQueueID; }
 
 	using iterator		 = typename std::vector<T>::iterator;
 	using const_iterator = typename std::vector<T>::const_iterator;
@@ -291,7 +295,7 @@ public:
 	bool		empty() const						{ return m_heap.empty(); }
 	T&			operator[](std::size_t idx) noexcept { assert(! address::is_padding(idx)); return m_heap[idx]; }
 	const T&    operator[](std::size_t idx) const noexcept { assert(! address::is_padding(idx)); return m_heap[idx]; }
-	static constexpr size_t invalid_id() { return std::numeric_limits<size_t>::max(); }
+    static constexpr size_t invalid_id() { return InvalidQueueID; }
 
 protected:
 	void		update_heap_up(size_t top, size_t bottom);
@@ -450,4 +454,6 @@ inline void MutableSkipHeapPriorityQueue<T, LessPredicate, IndexSetter, blocking
 	}
 }
 
+} // namespace Slic3r
+
 #endif /* slic3r_MutablePriorityQueue_hpp_ */
diff --git a/tests/libslic3r/test_mutable_priority_queue.cpp b/tests/libslic3r/test_mutable_priority_queue.cpp
index 626b0388d2..7e31b57c7c 100644
--- a/tests/libslic3r/test_mutable_priority_queue.cpp
+++ b/tests/libslic3r/test_mutable_priority_queue.cpp
@@ -4,6 +4,8 @@
 
 #include "libslic3r/MutablePriorityQueue.hpp"
 
+using namespace Slic3r;
+
 // based on https://raw.githubusercontent.com/rollbear/prio_queue/master/self_test.cpp
 // original source Copyright Björn Fahller 2015, Boost Software License, Version 1.0, http://www.boost.org/LICENSE_1_0.txt