diff --git a/BasiliskII/src/slirp/socket.c b/BasiliskII/src/slirp/socket.c
index dfa39b00..6968002e 100755
--- a/BasiliskII/src/slirp/socket.c
+++ b/BasiliskII/src/slirp/socket.c
@@ -15,6 +15,7 @@
 #endif
 #include <assert.h>
 #include <stdbool.h>
+#include <ctype.h>
 
 #define DEBUG_HOST_RESOLVED_DNS 0