diff --git a/Makefile.am b/Makefile.am
index 3dcc4ce..97b7c81 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -147,6 +147,7 @@ FULL_TESTS =                                    \
   tests/signals.org                             \
   tests/temporary-assignments.org               \
   tests/variable-and.org                        \
+  tests/variable-operators.org                  \
   tests/variable-or.org                         \
   tests/variable-patterns.org                   \
   tests/variable-words.org                      \
diff --git a/gash/word.scm b/gash/word.scm
index 7d6f8ac..e53a843 100644
--- a/gash/word.scm
+++ b/gash/word.scm
@@ -278,9 +278,15 @@ and arithmetic substitions."
              (setvar! name new-value)
              new-value))))
     (('<sh-ref-assert> name message)
-     (error (format #f "Not implemented: ~s" word)))
+     (or (parameter-ref name)
+         (let ((message* (expand-word message #:output 'string)))
+           (throw 'shell-error (format #f "~a: ~a" name message*)))))
     (('<sh-ref-assert*> name message)
-     (error (format #f "Not implemented: ~s" word)))
+     (let ((value (parameter-ref name)))
+       (if (string-not-null? value)
+           value
+           (let ((message* (expand-word message #:output 'string)))
+             (throw 'shell-error (format #f "~a: ~a" name message*))))))
     (('<sh-ref-and> name value)
      (if (string-not-null? (parameter-ref name))
          (word->qword (or value ""))
diff --git a/tests/variable-operators.org b/tests/variable-operators.org
new file mode 100644
index 0000000..11e5559
--- /dev/null
+++ b/tests/variable-operators.org
@@ -0,0 +1,26 @@
+;;; Gash -- Guile As SHell
+;;; Copyright © 2019 Timothy Sample <samplet@ngyro.com>
+;;;
+;;; This file is part of Gash.
+;;;
+;;; Gash is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Gash is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Gash.  If not, see <http://www.gnu.org/licenses/>.
+
+* Errors on failed asserting reference
+:script:
+#+begin_src sh
+  echo ${x?not set}
+  echo not shown
+#+end_src
+:status: 1
+:stdout: ""