diff --git a/src/racket/include/scheme.h b/src/racket/include/scheme.h
index 1a5a58ba37..d37b054b77 100644
--- a/src/racket/include/scheme.h
+++ b/src/racket/include/scheme.h
@@ -2002,10 +2002,10 @@ extern Scheme_Extension_Table *scheme_extension_table;
 # define MZ_FD_ISSET(n, p) scheme_fdisset(p, n)
 #else
 # define MZ_GET_FDSET(p, n) ((void *)(((fd_set *)p) + n))
-# define MZ_FD_ZERO(p) FD_ZERO(p)
-# define MZ_FD_SET(n, p) FD_SET(n, p)
-# define MZ_FD_CLR(n, p) FD_CLR(n, p)
-# define MZ_FD_ISSET(n, p) FD_ISSET(n, p)
+# define MZ_FD_ZERO(p) FD_ZERO((fd_set *)(p))
+# define MZ_FD_SET(n, p) FD_SET(n, (fd_set *)(p))
+# define MZ_FD_CLR(n, p) FD_CLR(n, (fd_set *)(p))
+# define MZ_FD_ISSET(n, p) FD_ISSET(n, (fd_set *)(p))
 #endif
 
 /* For scheme_fd_to_semaphore(): */