diff --git a/src/lib/fs.rs b/src/lib/fs.rs
index 06243b6d3aa..a2e81184d01 100644
--- a/src/lib/fs.rs
+++ b/src/lib/fs.rs
@@ -1,9 +1,10 @@
 
 import os::getcwd;
+import util::void;
 import os_fs;
 
-native "rust" mod rustrt {
-    fn rust_file_is_dir(path: str::sbuf) -> int;
+native "c-stack-cdecl" mod rustrt {
+    fn rust_file_is_dir(unused: *void, path: *u8) -> int;
 }
 
 fn path_sep() -> str { ret str::from_char(os_fs::path_sep); }
@@ -52,7 +53,9 @@ fn connect_many(paths: [path]) : vec::is_not_empty(paths) -> path {
 }
 
 fn file_is_dir(p: path) -> bool {
-    ret str::as_buf(p, {|buf| rustrt::rust_file_is_dir(buf) != 0 });
+    ret str::as_buf(p, {
+        |buf| rustrt::rust_file_is_dir(ptr::null(), buf) != 0
+    });
 }
 
 fn list_dir(p: path) -> [str] {
diff --git a/src/lib/util.rs b/src/lib/util.rs
index 1d9c28d99e4..eb1790f569d 100644
--- a/src/lib/util.rs
+++ b/src/lib/util.rs
@@ -17,6 +17,13 @@ pure fn rational_leq(x: rational, y: rational) -> bool {
 }
 
 pure fn orb(a: bool, b: bool) -> bool { a || b }
+
+// An unconstructible type. Currently we're using this for unused parameters
+// in native functions, but it may be useful for other purposes as well.
+tag void {
+    void(@void);
+}
+
 // Local Variables:
 // mode: rust;
 // fill-column: 78;