diff --git a/guix/utils.scm b/guix/utils.scm
index 5ec8f3736dbd870c103181ed50dd432957eaa995..453b3843ddff9e8774a25dd953d5f1bc61d5f8bd 100644
--- a/guix/utils.scm
+++ b/guix/utils.scm
@@ -156,7 +156,15 @@ (define sha256
 (define %nixpkgs-directory
   (make-parameter
    ;; Capture the build-time value of $NIXPKGS.
-   (or %nixpkgs (getenv "NIXPKGS"))))
+   (or %nixpkgs
+       (and=> (getenv "NIXPKGS")
+              (lambda (val)
+                ;; Bail out when passed an empty string, otherwise
+                ;; `nix-instantiate' will sit there and attempt to read
+                ;; from its standard input.
+                (if (string=? val "")
+                    #f
+                    val))))))
 
 (define* (nixpkgs-derivation attribute #:optional (system (%current-system)))
   "Return the derivation path of ATTRIBUTE in Nixpkgs."