diff --git a/src/racket/sconfig.h b/src/racket/sconfig.h
index dad9257be6..b8b21bfb5b 100644
--- a/src/racket/sconfig.h
+++ b/src/racket/sconfig.h
@@ -263,6 +263,8 @@
 # define USE_IEEE_FP_PREDS
 # define POW_HANDLES_INF_CORRECTLY
 
+# define USE_DYNAMIC_FDSET_SIZE
+
 # define SIGSET_IS_SIGNAL
 
 # define USE_TM_GMTOFF_FIELD