diff --git a/iverilog.conf b/iverilog.conf
index 7b97bac4e..71d1c048d 100644
--- a/iverilog.conf
+++ b/iverilog.conf
@@ -87,7 +87,7 @@
 # This is the XNF code generator.
 
 [-txnf]
-<ivl>%B/ivl %y %Y %[v-v] %s %[N-N%N] %[T-T%T] -txnf -Fsynth -Fsyn-rules -Fcprop -Fnodangle -o%o -- -
+<ivl>%B/ivl %y %Y %[v-v] %s %[N-N%N] %[T-T%T] -txnf -Fsynth -Fsyn-rules -Fxnfio -Fcprop -Fnodangle -o%o -- -
 
 # And this is another XNF code generator, under development.
 [-tfpga]