diff --git a/include/xo/expression/Constant.hpp b/include/xo/expression/Constant.hpp
index 77098a5b..b2c9f71d 100644
--- a/include/xo/expression/Constant.hpp
+++ b/include/xo/expression/Constant.hpp
@@ -66,10 +66,13 @@ namespace xo {
             }
 
             virtual void display(std::ostream & os) const override {
-                os << "<Constant"
-                   << xtag("type", value_td_->short_name())
-                   << xtag("value", value_)
-                   << ">";
+                os << "<Constant";
+                if (value_td_)
+                    os << xtag("type", value_td_->short_name());
+                else
+                    os << xtag("type", "nullptr");;
+                os << xtag("value", value_);
+                os << ">";
             }
 
         private:
diff --git a/src/expression/Variable.cpp b/src/expression/Variable.cpp
index 766e0c67..18434afb 100644
--- a/src/expression/Variable.cpp
+++ b/src/expression/Variable.cpp
@@ -18,9 +18,12 @@ namespace xo {
         void
         Variable::display(std::ostream & os) const {
             os << "<Variable"
-               << xtag("name", name_)
-               << xtag("type", this->valuetype()->short_name())
-               << ">";
+               << xtag("name", name_);
+            if (this->valuetype())
+                os << xtag("type", this->valuetype()->short_name());
+            else
+                os << xtag("type", "nullptr");
+            os << ">";
         } /*display*/
     } /*namespace ast*/
 } /*namespace xo*/