diff --git a/core/src/nix/drv.sld b/core/src/nix/drv.sld
index a8b69d6..5234f60 100644
--- a/core/src/nix/drv.sld
+++ b/core/src/nix/drv.sld
@@ -234,14 +234,16 @@
             (error "Derivation output path mismatch: " (make-output-path "sha256" modulo-hash (car output) name) " vs " (derivation-output-path (cdr output)))))
         (derivation-outputs orig-drv)))
 
+    (define (filter-environment env-list output)
+      (for-each (lambda (kv) (unless (assoc (car kv) output string=?) (set! output (cons kv output)))) env-list)
+      output)
+
     ;; Creates an input-addressed derivation with specified parameters.
     (define (make-input-addressed-derivation name platform input-drvs input-srcs builder env outputs)
       (define compat-env (if (%derivation-compatible) `(("name" . ,name) ("builder" . ,(car builder)) ("system" . ,platform)) '()))
 
       (define tmp-outputs (list-sort env-pair< (map (lambda (l) (cons l (make-derivation-output #f #f #f #f))) outputs)))
-      (define tmp-env (list-sort env-pair< (apply append (list (map (lambda (l) (cons l "")) outputs)
-                                                               compat-env
-                                                               env))))
+      (define tmp-env (list-sort env-pair< (filter-environment env (append compat-env (map (lambda (l) (cons l "")) outputs)))))
 
       (define tmp-drv (make-derivation name tmp-outputs input-drvs input-srcs platform (car builder) (cdr builder) tmp-env (make-derivation-cached-data #f #f #f #f)))
       (define modulo-hash (modulo-hash-drv tmp-drv))
@@ -249,7 +251,7 @@
 
       (define new-outputs (list-sort env-pair< (map (lambda (l) (cons l (make-derivation-output
                                                                          (if is-deferred #f (make-output-path "sha256" modulo-hash l name)) #f #f #f))) outputs)))
-      (define new-env (list-sort env-pair< (apply append (list (map (lambda (l) (cons l (make-output-path "sha256" modulo-hash l name))) outputs) compat-env env))))
+      (define new-env (list-sort env-pair< (filter-environment env (append compat-env (map (lambda (l) (cons l (make-output-path "sha256" modulo-hash l name))) outputs)))))
 
       (define drv (make-derivation name new-outputs input-drvs input-srcs platform (car builder) (cdr builder) new-env (make-derivation-cached-data #f #f (derivation-cached-data-is-deferred (derivation-cached-data tmp-drv)) #f)))
       (sanity-check-drv drv)
@@ -261,7 +263,7 @@
 
       (define outputs (list-sort env-pair< (map (lambda (l) (cons l (make-derivation-output
                                                                      #f 'impure "sha256" #t))) noutputs)))
-      (define env (list-sort env-pair< (apply append (list (map (lambda (l) (cons (car l) (make-placeholder (car l)))) outputs) compat-env nenv))))
+      (define env (list-sort env-pair< (filter-environment env (append compat-env (map (lambda (l) (cons (car l) (make-placeholder (car l)))) outputs)))))
 
       (define drv (make-derivation name outputs input-drvs input-srcs platform (car builder) (cdr builder) env (make-derivation-cached-data #f #f #t #f)))
       (define pathhash (string-copy (derivation-path drv) (+ 1 (string-length (%store-dir))) (+ 33 (string-length (%store-dir)))))
@@ -274,7 +276,7 @@
 
       (define outputs (list-sort env-pair< (map (lambda (l) (cons l (make-derivation-output
                                                                      #f 'floating "sha256" #t))) noutputs)))
-      (define env (list-sort env-pair< (apply append (list (map (lambda (l) (cons (car l) (make-placeholder (car l)))) outputs) compat-env nenv))))
+      (define env (list-sort env-pair< (filter-environment nenv (append compat-env (map (lambda (l) (cons (car l) (make-placeholder (car l)))) outputs)))))
 
       (define drv (make-derivation name outputs input-drvs input-srcs platform (car builder) (cdr builder) env (make-derivation-cached-data #f #f #t #f)))
       (define pathhash (string-copy (derivation-path drv) (+ 1 (string-length (%store-dir))) (+ 33 (string-length (%store-dir)))))