diff --git a/guix/scripts/offload.scm b/guix/scripts/offload.scm
index 2d2dbe36c5b84253e5b2a9874467e3641154583f..e1da31af5d3df05c524bbf1837996b4a7782f6e4 100644
--- a/guix/scripts/offload.scm
+++ b/guix/scripts/offload.scm
@@ -383,6 +383,10 @@ (define (release-build-slot slot)
   "Release SLOT, a build slot as returned as by 'acquire-build-slot'."
   (close-port slot))
 
+(define %slots
+  ;; List of acquired build slots (open ports).
+  '())
+
 (define (choose-build-machine requirements machines)
   "Return the best machine among MACHINES fulfilling REQUIREMENTS, or #f."
 
@@ -418,7 +422,10 @@ (define (undecorate pred)
 
          ;; Return the best machine unless it's already overloaded.
          (if (< (machine-load best) 2.)
-             best
+             (begin
+               ;; Prevent SLOT from being GC'd.
+               (set! %slots (cons slot %slots))
+               best)
              (begin
                (release-build-slot slot)
                #f)))