diff --git a/src/include/config.h.in b/src/include/config.h.in
index 85cf00112d4..e1b9bf8482e 100644
--- a/src/include/config.h.in
+++ b/src/include/config.h.in
@@ -96,13 +96,15 @@
  * Maximum number of columns in an index and maximum number of arguments
  * to a function. They must be the same value.
  *
+ * The minimum value is 9 (btree index creation has a 9-argument function).
+ *
  * There is no maximum value, though if you want to pass more than 32 
  * arguments to a function, you will have to modify 
  * pgsql/src/backend/utils/fmgr/fmgr.c and add additional entries 
  * to the 'case' statement for the additional arguments.
  */
 #define INDEX_MAX_KEYS		16
-#define FUNC_MAX_ARGS		(INDEX_MAX_KEYS+1)
+#define FUNC_MAX_ARGS		INDEX_MAX_KEYS
 
 /*
  * Enables debugging print statements in the date/time support routines.