diff --git a/lang/ninja/src/nixpkgs.sld b/lang/ninja/src/nixpkgs.sld
index 1c2510f..7c8fe92 100644
--- a/lang/ninja/src/nixpkgs.sld
+++ b/lang/ninja/src/nixpkgs.sld
@@ -377,6 +377,8 @@
       rewrites))
     (make-finalized-drv store-paths export-depfile secondary-roots conf))
   
+  (define marker-header (string->utf8 "ZILCH MARKER FILE ->"))
+
   ;; Build the derivation, but with stubbed out header and .so files.
   ;; This is used to determine the dataflow, to make cross-project incremental
   ;; builds work.
@@ -469,8 +471,8 @@
     (define (get-file-marker fptr)
       (call-with-port (store-path-open fptr)
        (lambda (p)
-        (define header (read-string 20 p))
-        (and (string=? header "ZILCH MARKER FILE ->")
+        (define header (read-bytevector 20 p))
+        (and (equal? header marker-header)
          (let ((str (read-string 99999 p)))
            (string-copy str 0 (- (string-length str) 1)))))))
     (define output (mapping (make-default-comparator)))
@@ -489,5 +491,3 @@
        (vfs-contents vfs)))
     (for-each process-output patched-drv)
     output)))
-
-    ; TODO(puck): for each output, do the necessary dance of figuring out where it came from. read first N bytes, compare, then do the thing. output a big alist and do the dataflow dance?