diff --git a/tools/nfdc/format-helpers.cpp b/tools/nfdc/format-helpers.cpp
index d898dc3..fa72512 100644
--- a/tools/nfdc/format-helpers.cpp
+++ b/tools/nfdc/format-helpers.cpp
@@ -192,6 +192,12 @@
   return os << (v.flag ? "on" : "off");
 }
 
+std::ostream&
+operator<<(std::ostream& os, YesNo v)
+{
+  return os << (v.flag ? "yes" : "no");
+}
+
 std::string
 formatTimestamp(time::system_clock::TimePoint t)
 {