diff --git a/lang/ninja/src/build.sld b/lang/ninja/src/build.sld
index 90496ad..461d305 100644
--- a/lang/ninja/src/build.sld
+++ b/lang/ninja/src/build.sld
@@ -120,7 +120,7 @@
   ;; to be a nixpkgs-style derivation, the same way `nix-shell` works.
   (define (process-ninja-file file vfs environment relative-to)
     (when (or (derivation? environment) (store-path? environment)) (set! environment (environment-for-derivation environment)))
-    (unless (string-suffix? "/" relative-to) (set! relative-to (string-append relative-to "/")))
+    (unless (or (string=? relative-to "") (string-suffix? "/" relative-to)) (set! relative-to (string-append relative-to "/")))
 
     (define edges (mapping (make-default-comparator)))