diff --git a/src/hardware/softmodem.cpp b/src/hardware/softmodem.cpp
index b05102e2..ad11e532 100644
--- a/src/hardware/softmodem.cpp
+++ b/src/hardware/softmodem.cpp
@@ -23,6 +23,7 @@
 #if C_MODEM
 
 #include <string.h>
+#include <ctype.h>
 #include "SDL_net.h"
 
 #include "inout.h"