diff --git a/src/ast_values.cpp b/src/ast_values.cpp
index ae5aa6c..a381c1a 100644
--- a/src/ast_values.cpp
+++ b/src/ast_values.cpp
@@ -33,6 +33,8 @@
 {
 }
 
+empty_::empty_() = default;
+
 bool module_::operator<(const module_& b) const
 {
     return this->m_name < b.m_name;
@@ -58,6 +60,16 @@
     return this->m_value < b.m_value;
 }
 
+bool empty_::operator==(const empty_) const
+{
+    return true;
+}
+
+bool empty_::operator<(const empty_) const
+{
+    return false;
+}
+
 bool enum_::operator==(const enum_& b) const
 {
     return this->m_value == b.m_value;