diff --git a/src/address_dictionary.h b/src/address_dictionary.h
index 5a4ab7ef..0a408921 100644
--- a/src/address_dictionary.h
+++ b/src/address_dictionary.h
@@ -23,6 +23,8 @@ extern "C" {
 
 #define DEFAULT_ADDRESS_EXPANSION_PATH LIBPOSTAL_DATA_DIR PATH_SEPARATOR "address_expansions" PATH_SEPARATOR "address_dictionary.dat"
 
+#define NULL_CANONICAL_INDEX -1
+
 typedef union expansion_value {
     uint32_t value;
     struct {
@@ -76,4 +78,4 @@ void address_dictionary_module_teardown(void);
 }
 #endif
 
-#endif
\ No newline at end of file
+#endif