diff --git a/src/who.c b/src/who.c
index fa3e21104..184a8cb70 100644
--- a/src/who.c
+++ b/src/who.c
@@ -33,6 +33,7 @@
 
 #include "readutmp.h"
 #include "error.h"
+#include "vasprintf.h"
 
 /* The official name of this program (e.g., no `g' prefix).  */
 #define PROGRAM_NAME "who"