diff --git a/src/lib/fs.rs b/src/lib/fs.rs
index 0e9dc591756b..922d01e841be 100644
--- a/src/lib/fs.rs
+++ b/src/lib/fs.rs
@@ -4,7 +4,7 @@ import os_fs;
 import str;
 
 native "rust" mod rustrt {
-    fn rust_file_is_dir(path: str) -> int;
+    fn rust_file_is_dir(path: istr::sbuf) -> int;
 }
 
 fn path_sep() -> istr { ret istr::from_char(os_fs::path_sep); }
@@ -43,7 +43,9 @@ fn connect(pre: &path, post: &path) -> path {
 }
 
 fn file_is_dir(p: &path) -> bool {
-    ret rustrt::rust_file_is_dir(istr::to_estr(p)) != 0;
+    ret istr::as_buf(p, { |buf|
+        rustrt::rust_file_is_dir(buf) != 0
+    });
 }
 
 fn list_dir(p: &path) -> [istr] {
diff --git a/src/rt/rust_builtin.cpp b/src/rt/rust_builtin.cpp
index 3e89d12e4382..34624f1a8171 100644
--- a/src/rt/rust_builtin.cpp
+++ b/src/rt/rust_builtin.cpp
@@ -444,9 +444,9 @@ rust_dirent_filename(rust_task *task, dirent* ent) {
 #endif
 
 extern "C" CDECL int
-rust_file_is_dir(rust_task *task, rust_str *path) {
+rust_file_is_dir(rust_task *task, char *path) {
     struct stat buf;
-    if (stat((char*)path->data, &buf)) {
+    if (stat(path, &buf)) {
         return 0;
     }
     return S_ISDIR(buf.st_mode);