diff --git a/.gitignore b/.gitignore
index 5a1b4f1..a6047cb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@ diagnostic.test
 tests/*.diff
 spec/
 spec-runner/
+node_modules/
diff --git a/mo b/mo
index b143808..fc168b5 100755
--- a/mo
+++ b/mo
@@ -13,9 +13,10 @@
 #/
 #/    mo [--false] [--help] [--source=FILE] filenames...
 #/
-#/ --false       - Treat the string "false" as empty for conditionals.
-#/ --help        - This message.
-#/ --source=FILE - Load FILE into the environment before processing templates.
+#/ --fail-not-set - Fail upon expansion of an unset variable.
+#/ --false        - Treat the string "false" as empty for conditionals.
+#/ --help         - This message.
+#/ --source=FILE  - Load FILE into the environment before processing templates.
 #
 # Mo is under a MIT style licence with an additional non-advertising clause.
 # See LICENSE.md for the full text.
@@ -27,19 +28,23 @@
 
 # Public: Template parser function.  Writes templates to stdout.
 #
-# $0            - Name of the mo file, used for getting the help message.
-# --false       - Treat "false" as an empty value.  You may set the
-#                 MO_FALSE_IS_EMPTY environment variable instead to a non-empty
-#                 value to enable this behavior.
-# --help        - Display a help message.
-# --source=FILE - Source a file into the environment before processint
-#                 template files.
-# --            - Used to indicate the end of options.  You may optionally
-#                 use this when filenames may start with two hyphens.
-# $@            - Filenames to parse.
+# $0             - Name of the mo file, used for getting the help message.
+# --fail-not-set - Fail upon expansion of an unset variable.  Default behavior
+#                  is to silently ignore and expand into empty string.
+# --false        - Treat "false" as an empty value.  You may set the
+#                  MO_FALSE_IS_EMPTY environment variable instead to a non-empty
+#                  value to enable this behavior.
+# --help         - Display a help message.
+# --source=FILE  - Source a file into the environment before processint
+#                  template files.
+# --             - Used to indicate the end of options.  You may optionally
+#                  use this when filenames may start with two hyphens.
+# $@             - Filenames to parse.
 #
 # Mo uses the following environment variables:
 #
+# MO_FAIL_ON_UNSET    - When set to a non-empty value, expansion of an unset
+#                       env variable will be aborted with an error.
 # MO_FALSE_IS_EMPTY   - When set to a non-empty value, the string "false"
 #                       will be treated as an empty value for the purposes
 #                       of conditionals.
@@ -69,6 +74,11 @@ mo() (
                         exit 0
                         ;;
 
+                    --fail-not-set)
+                        # shellcheck disable=SC2030
+                        MO_FAIL_ON_UNSET=true
+                        ;;
+
                     --false)
                         # shellcheck disable=SC2030
                         MO_FALSE_IS_EMPTY=true
@@ -255,7 +265,7 @@ moIndentLines() {
     local content fragment len posN posR result trimmed
 
     result=""
-    
+
     #: Remove the period from the end of the string.
     len=$((${#3} - 1))
     content=${3:0:$len}
@@ -746,7 +756,13 @@ moShow() {
             eval moJoin moJoined "," "\${$1[@]}"
             echo -n "$moJoined"
         else
-            echo -n "${!1}"
+            # shellcheck disable=SC2031
+            if [[ -z "$MO_FAIL_ON_UNSET" ]] || moTest "$1"; then
+                echo -n "${!1}"
+            else
+                echo "Env variable not set: $1" >&2
+                exit 1
+            fi
         fi
     else
         # Further subindexes are disallowed
diff --git a/tests/fail-not-set.expected b/tests/fail-not-set.expected
new file mode 100644
index 0000000..06dea8f
--- /dev/null
+++ b/tests/fail-not-set.expected
@@ -0,0 +1 @@
+This will fail: Env variable not set: __NO_SUCH_VAR
diff --git a/tests/fail-not-set.sh b/tests/fail-not-set.sh
new file mode 100755
index 0000000..d3f9ef7
--- /dev/null
+++ b/tests/fail-not-set.sh
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+cd "${0%/*}"
+unset __NO_SUCH_VAR
+echo "This will fail: {{__NO_SUCH_VAR}}" | ../mo --fail-not-set 2>&1
+
+if [[ $? -ne 1 ]]; then
+    echo "Did not return 1"
+fi
diff --git a/tests/help.expected b/tests/help.expected
index 95e8df5..51a740a 100644
--- a/tests/help.expected
+++ b/tests/help.expected
@@ -11,6 +11,7 @@ Simple usage:
 
    mo [--false] [--help] [--source=FILE] filenames...
 
---false       - Treat the string "false" as empty for conditionals.
---help        - This message.
---source=FILE - Load FILE into the environment before processing templates.
+--fail-not-set - Fail upon expansion of an unset variable.
+--false        - Treat the string "false" as empty for conditionals.
+--help         - This message.
+--source=FILE  - Load FILE into the environment before processing templates.