diff --git a/src/mzscheme/dynsrc/ustart.c b/src/mzscheme/dynsrc/ustart.c
index 710a5ddadb..4d1d14d7f6 100644
--- a/src/mzscheme/dynsrc/ustart.c
+++ b/src/mzscheme/dynsrc/ustart.c
@@ -78,6 +78,7 @@ static X_flag_entry X_flags[] = {
   { "-title", 1 },
   { "-xnllanguage", 1 },
   { "-xrm", 1 },
+  { "-singleInstance", 0 },
   { NULL, 0 }
 };