acos(3) returns -nan instead of nan

Index: test/parser.fnl
--- test/parser.fnl.orig
+++ test/parser.fnl
@@ -48,7 +48,7 @@
        (fennel.view (fennel.eval ".inf")))
   (t.= "-.inf"
        (fennel.view (fennel.eval "-.inf")))
-  (t.= ".nan"
+  (t.= "-.nan"
        (fennel.view (fennel.eval "(math.acos 2)")))
   (t.= ".nan"
        (fennel.view (fennel.eval ".nan")))
