diff --git a/guix/build-system/gnu.scm b/guix/build-system/gnu.scm
index c67ec523427f73244f06493f11a9a6888c803d2b..994b386e5c38c4569e49278ebcca3a5cbcac9095 100644
--- a/guix/build-system/gnu.scm
+++ b/guix/build-system/gnu.scm
@@ -120,12 +120,21 @@ (define* (gnu-build store name source inputs
                     (phases '%standard-phases)
                     (system (%current-system))
                     (implicit-inputs? #t)    ; useful when bootstrapping
+                    (imported-modules '((guix build gnu-build-system)
+                                        (guix build utils)))
                     (modules '((guix build gnu-build-system)
                                (guix build utils))))
   "Return a derivation called NAME that builds from tarball SOURCE, with
 input derivation INPUTS, using the usual procedure of the GNU Build
 System.  The builder is run with GUILE, or with the distro's final Guile
-package if GUILE is #f or omitted."
+package if GUILE is #f or omitted.
+
+The builder is run in a context where MODULES are used; IMPORTED-MODULES
+specifies modules not provided by Guile itself that must be imported in
+the builder's environment, from the host.  Note that we distinguish
+between both, because for Guile's own modules like (ice-9 foo), we want
+to use GUILE's own version of it, rather than import the user's one,
+which could lead to gratuitous input divergence."
   (define builder
     `(begin
        (use-modules ,@modules)
@@ -170,7 +179,7 @@ (define guile-for-build
                                           (standard-inputs system))
                                         '()))
                                 #:outputs outputs
-                                #:modules modules
+                                #:modules imported-modules
                                 #:guile-for-build guile-for-build))
 
 (define gnu-build-system