diff --git a/src/num_to_char.c b/src/num_to_char.c
index e5218e1..941a9dc 100644
--- a/src/num_to_char.c
+++ b/src/num_to_char.c
@@ -1,5 +1,6 @@
 #include <Rdefines.h>
 #include <Rinternals.h>
+#include <math.h>
 #include <stdlib.h>
 #include "modp_numtoa.h"