diff --git a/extractor/backoff_sampler.cc b/extractor/backoff_sampler.cc
index 891276c63..78131a9a3 100644
--- a/extractor/backoff_sampler.cc
+++ b/extractor/backoff_sampler.cc
@@ -2,6 +2,7 @@
 
 #include "data_array.h"
 #include "phrase_location.h"
+#include <cmath>
 
 namespace extractor {