diff --git a/gtsam/inference/Key.cpp b/gtsam/inference/Key.cpp
index 8fcea0d05..15d633eeb 100644
--- a/gtsam/inference/Key.cpp
+++ b/gtsam/inference/Key.cpp
@@ -10,7 +10,7 @@
  * -------------------------------------------------------------------------- */
 
 /**
- * @file Key.h
+ * @file Key.cpp
  * @brief
  * @author Richard Roberts
  * @author Alex Cunningham
@@ -26,6 +26,9 @@ using namespace std;
 
 namespace gtsam {
 
+/// Assign default key formatter
+KeyFormatter DefaultKeyFormatter = &_defaultKeyFormatter;
+
 /* ************************************************************************* */
 string _defaultKeyFormatter(Key key) {
   const Symbol asSymbol(key);
diff --git a/gtsam/inference/Key.h b/gtsam/inference/Key.h
index 31428a50e..a02d018f5 100644
--- a/gtsam/inference/Key.h
+++ b/gtsam/inference/Key.h
@@ -37,10 +37,16 @@ using KeyFormatter = std::function<std::string(Key)>;
 // Helper function for DefaultKeyFormatter
 GTSAM_EXPORT std::string _defaultKeyFormatter(Key key);
 
-/// The default KeyFormatter, which is used if no KeyFormatter is passed to
-/// a nonlinear 'print' function.  Automatically detects plain integer keys
-/// and Symbol keys.
-static const KeyFormatter DefaultKeyFormatter = &_defaultKeyFormatter;
+/**
+ * The default KeyFormatter, which is used if no KeyFormatter is passed
+ * to a 'print' function.
+ *
+ * Automatically detects plain integer keys and Symbol keys.
+ * 
+ * Marked as `extern` so that it can be updated by external libraries.
+ *
+ */
+extern GTSAM_EXPORT KeyFormatter DefaultKeyFormatter;
 
 // Helper function for Multi-robot Key Formatter
 GTSAM_EXPORT std::string _multirobotKeyFormatter(gtsam::Key key);
@@ -124,7 +130,3 @@ struct traits<Key> {
 };
 
 } // namespace gtsam
-
-
-
-