diff --git a/core/src/file.sld b/core/src/file.sld
index a1b3131..80bd0f4 100644
--- a/core/src/file.sld
+++ b/core/src/file.sld
@@ -216,6 +216,7 @@
             (zexp (("src" . ".attr-1s42g1c76fxb77skzq0b4wdhcrg8jmzb54czmxvh1qm7psgsbcni")
                    ("contents" . (zexp-unquote (call-with-port (open-output-bytevector) (lambda (port) (parameterize ((current-output-port port)) (serialize-as-tar val "-")) (get-output-bytevector port)))))
                    ("passAsFile" . "contents")
+                   ("allowSubstitutes" . "")
                    ("channelName" . "-")))
             '("out")))
         (cdar drv))))