diff --git a/alpha_encoder/alpha_encoder.cc b/alpha_encoder/alpha_encoder.cc
index 4402ab1..74feddb 100644
--- a/alpha_encoder/alpha_encoder.cc
+++ b/alpha_encoder/alpha_encoder.cc
@@ -17,6 +17,7 @@
  */
 
 #include <cstdio>
+#include <cstdlib>
 #include <cstring>
 #include <sstream>
 #include <string>