diff --git a/src/mzscheme/src/port.c b/src/mzscheme/src/port.c
index d83c121f12..eb89a167fd 100644
--- a/src/mzscheme/src/port.c
+++ b/src/mzscheme/src/port.c
@@ -2769,7 +2769,8 @@ static int do_peekc_skip(Scheme_Object *port, Scheme_Object *skip,
 				      NULL);
 
     if (!v) {
-      *unavail = 1;
+      if (unavail)
+        *unavail = 1;
       return 0;
     }