diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index a1a93ad..c978d76 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -26,7 +26,9 @@
  *
  * Changing this requires an initdb.
  */
+#ifndef NAMEDATALEN
 #define NAMEDATALEN 64
+#endif

 /*
  * Maximum number of arguments to a function.