diff --git a/fp.c b/fp.c
index 7d20175..0eb4191 100644
--- a/fp.c
+++ b/fp.c
@@ -227,7 +227,7 @@ size_t fpgdec64(unsigned char *in, size_t n, uint64_t *out, uint64_t start) { if
 // Up to 300 times better compression and several times faster
 #define N2  6 // for seconds time series
 #define N3 10
-#define N4 17  
+#define N4 17 // must be > 16 
 #define NL 18
 
 #define ENC32(_pp_, _ip_, _d_, _op_) do {\