diff --git a/lalrpop/src/build/mod.rs b/lalrpop/src/build/mod.rs
index a78a1af..096f577 100644
--- a/lalrpop/src/build/mod.rs
+++ b/lalrpop/src/build/mod.rs
@@ -33,10 +33,11 @@ fn process_dir<P:AsRef<Path>>(root_dir: P, force_build: bool) -> io::Result<()>
     for lalrpop_file in lalrpop_files {
         let rs_file = lalrpop_file.with_extension("rs");
         if force_build || try!(needs_rebuild(&lalrpop_file, &rs_file)) {
+            try!(make_read_only(&rs_file, false));
             try!(remove_old_file(&rs_file));
             let grammar = try!(parse_and_normalize_grammar(lalrpop_file));
             try!(emit_recursive_ascent(&rs_file, &grammar));
-            try!(make_read_only(&rs_file));
+            try!(make_read_only(&rs_file, true));
         }
     }
     Ok(())
@@ -99,11 +100,15 @@ fn needs_rebuild(lalrpop_file: &Path,
     }
 }
 
-fn make_read_only(rs_file: &Path) -> io::Result<()> {
-    let rs_metadata = try!(fs::metadata(&rs_file));
-    let mut rs_permissions = rs_metadata.permissions();
-    rs_permissions.set_readonly(true);
-    fs::set_permissions(&rs_file, rs_permissions)
+fn make_read_only(rs_file: &Path, ro: bool) -> io::Result<()> {
+    if rs_file.is_file() {
+        let rs_metadata = try!(fs::metadata(&rs_file));
+        let mut rs_permissions = rs_metadata.permissions();
+        rs_permissions.set_readonly(ro);
+        fs::set_permissions(&rs_file, rs_permissions)
+    } else {
+        Ok(())
+    }
 }
 
 fn lalrpop_files<P:AsRef<Path>>(root_dir: P) -> io::Result<Vec<PathBuf>> {