diff --git a/include/spiffing/classification.h b/include/spiffing/classification.h
index f8fb1c3..fb8f42c 100644
--- a/include/spiffing/classification.h
+++ b/include/spiffing/classification.h
@@ -88,7 +88,7 @@ namespace Spiffing {
 	};
 
 	struct ClassificationHierarchyCompare {
-		bool operator() (std::shared_ptr<Classification> const & l, std::shared_ptr<Classification> const & r) {
+		bool operator() (std::shared_ptr<Classification> const & l, std::shared_ptr<Classification> const & r) const {
 			return l->hierarchy() < r->hierarchy();
 		}
 	};
diff --git a/include/spiffing/tagset.h b/include/spiffing/tagset.h
index 3b3775f..f21dd52 100644
--- a/include/spiffing/tagset.h
+++ b/include/spiffing/tagset.h
@@ -32,6 +32,7 @@ SOFTWARE.
 #include <memory>
 #include <map>
 #include <set>
+#include <stdexcept>
 
 namespace Spiffing {
     class TagSet {