diff --git a/mpact/sim/decoder/slot.cc b/mpact/sim/decoder/slot.cc
index c3a80b6..bb348c8 100644
--- a/mpact/sim/decoder/slot.cc
+++ b/mpact/sim/decoder/slot.cc
@@ -134,6 +134,8 @@
                           : "))");
 }
 
+absl::btree_set<std::string> Slot::attribute_names_;
+
 Slot::Slot(absl::string_view name, InstructionSet* instruction_set,
            bool is_templated, SlotDeclCtx* ctx, unsigned generator_version)
     : instruction_set_(instruction_set),
diff --git a/mpact/sim/decoder/slot.h b/mpact/sim/decoder/slot.h
index 640ff0c..6ebc0df 100644
--- a/mpact/sim/decoder/slot.h
+++ b/mpact/sim/decoder/slot.h
@@ -265,7 +265,8 @@
   absl::btree_map<std::string, IdentListCtx*> resource_array_ref_map_;
   // Default instruction attributes.
   absl::btree_map<std::string, TemplateExpression*> attribute_map_;
-  absl::btree_set<std::string> attribute_names_;
+  // We are using a global set of attritube names so make this static.
+  static absl::btree_set<std::string> attribute_names_;
 };
 
 }  // namespace instruction_set