diff --git a/src/libcore/os.rs b/src/libcore/os.rs
index 98bac3a49d8..8a512da7dc0 100644
--- a/src/libcore/os.rs
+++ b/src/libcore/os.rs
@@ -374,6 +374,7 @@ pub fn self_exe_path() -> Option<Path> {
                 let mib = ~[CTL_KERN as c_int,
                            KERN_PROC as c_int,
                            KERN_PROC_PATHNAME as c_int, -1 as c_int];
+                let mut sz = sz;
                 sysctl(vec::raw::to_ptr(mib), vec::len(mib) as c_uint,
                        buf as *mut c_void, &mut sz, ptr::null(),
                        0u as size_t) == (0 as c_int)