diff --git a/tests/spec/oil.scm b/tests/spec/oil.scm
index cbc9c6f..1f07436 100644
--- a/tests/spec/oil.scm
+++ b/tests/spec/oil.scm
@@ -50,10 +50,15 @@
            (substitute* "test/common.sh"
              (("/usr/bin/env time") (which "time")))
 
-           ;; We do not have the BusyBox shell, but the test suite
-           ;; will try to use it anyway.  This change fixes that.
+           ;; Disable all shells except for Bash.  This is mostly for
+           ;; reproducibility, since it is annoying when the tests
+           ;; fail because a different set of reference shells are
+           ;; available.
            (substitute* "test/spec.sh"
-             (("BUSYBOX_ASH=_tmp/shells/ash") "BUSYBOX_ASH="))
+             (("BUSYBOX_ASH=_tmp/shells/ash") "BUSYBOX_ASH=")
+             (("readonly DASH=\\$[(]shell-path dash[)]") "readonly DASH=")
+             (("readonly MKSH=\\$[(]shell-path mksh[)]") "readonly MKSH=")
+             (("readonly ZSH=\\$[(]shell-path zsh[)]") "readonly ZSH="))
 
            ;; This is not necessary, but it makes the output nicer.
            (substitute* "test/spec.sh"