diff --git a/test-suite/tests/posix.test b/test-suite/tests/posix.test
index d8975f02a..64b8d96af 100644
--- a/test-suite/tests/posix.test
+++ b/test-suite/tests/posix.test
@@ -171,11 +171,13 @@
 
   (pass-if-exception "non-tty argument" exception:system-error
     ;; This used to crash in 1.8.1 and earlier.
-    (let ((file (false-if-exception
-                 (open-output-file "/dev/null"))))
-      (if (not file)
-          (throw 'unsupported)
-          (ttyname file)))))
+    (if (not (defined? 'ttyname))
+        (throw 'unsupported)
+        (let ((file (false-if-exception
+                     (open-output-file "/dev/null"))))
+          (if (not file)
+              (throw 'unsupported)
+              (ttyname file))))))
 
 ;;
 ;; utimes