diff --git a/src/libslic3r/Fill/FillBase.cpp b/src/libslic3r/Fill/FillBase.cpp
index 2b307007b8..f4615dc640 100644
--- a/src/libslic3r/Fill/FillBase.cpp
+++ b/src/libslic3r/Fill/FillBase.cpp
@@ -2467,9 +2467,11 @@ void Fill::connect_base_support(Polylines &&infill_ordered, const std::vector<co
 #endif // INFILL_DEBUG_OUTPUT
 
     const std::vector<SupportArcCost> arches = evaluate_support_arches(infill_ordered, graph, spacing, params);
-    static const double cost_low      = line_spacing * 1.3;
-    static const double cost_high     = line_spacing * 2.;
-    static const double cost_veryhigh = line_spacing * 3.;
+    // Must not be static: line_spacing varies per call (base vs interface fills differ),
+    // and a static here would fix these to whichever call ran first, order depending on thread count.
+    const double cost_low      = line_spacing * 1.3;
+    const double cost_high     = line_spacing * 2.;
+    const double cost_veryhigh = line_spacing * 3.;
 
     {
         std::vector<const SupportArcCost*> selected;