diff --git a/toolchains/xslt-M4/testing/issue_235_regression-metaschema.xml b/toolchains/xslt-M4/testing/issue_235_regression-metaschema.xml
new file mode 100644
index 000000000..01e13836b
--- /dev/null
+++ b/toolchains/xslt-M4/testing/issue_235_regression-metaschema.xml
@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<METASCHEMA xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0">
+    <schema-name>issue-235-test-schema</schema-name>
+    <schema-version>0.1</schema-version>
+    <short-name>oscal-value-testing-mini</short-name>
+    <namespace>http://csrc.nist.gov/ns/metaschema/unit-tests</namespace>
+    <json-base-uri>http://csrc.nist.gov/ns/oscal</json-base-uri>
+    <define-assembly name="root-assembly">
+        <formal-name>Issue 235 Regression Test</formal-name>
+        <description>A sample Metaschema definition for <a href="https://github.com/usnistgov/metaschema/issues/235">regression testing</a>.</description>
+        <root-name>root</root-name>
+        <model>
+            <define-field name="field1" as-type="uri">
+                <formal-name>Field 1</formal-name>
+                <description>An example field with a flag constrained with optional values.</description>
+                <!--<json-value-key>id</json-value-key>-->
+                <define-flag name="optional-flag" as-type="string">
+                    <constraint>
+                        <allowed-values allow-other="yes">
+                            <enum value="value1"/>
+                            <enum value="value2"/>
+                        </allowed-values>
+                    </constraint>
+                </define-flag>
+            </define-field>
+        </model>
+    </define-assembly>
+</METASCHEMA>
diff --git a/toolchains/xslt-M4/testing/issue_235_regression.xspec b/toolchains/xslt-M4/testing/issue_235_regression.xspec
new file mode 100644
index 000000000..dc6fe0396
--- /dev/null
+++ b/toolchains/xslt-M4/testing/issue_235_regression.xspec
@@ -0,0 +1,144 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<x:description 
+    xmlns:x="http://www.jenitennison.com/xslt/xspec"
+    xmlns:mv="http://g"
+    stylesheet="../converter-gen/supermodel-to-json.xsl">
+    <x:scenario label="Regression Test for usnistgov/metaschema#235">
+        <x:scenario label="If a document instance has a field has no value or flag defined">
+            <x:context  mode="write-json">
+                <!--
+                    <root xmlns="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                        <field1 optional-flag="value2">https://example.com</field1>
+                    </root>
+                -->
+                <assembly xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0/supermodel"
+                    name="root-assembly"
+                    key="root"
+                    gi="root"
+                    namespace="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                    <field collapsible="no"
+                        as-type="uri"
+                        name="field1"
+                        key="field1"
+                        gi="field1">
+                        <value as-type="uri" key="STRVALUE" in-json="string"/>
+                    </field>
+                </assembly>
+            </x:context>
+            <x:expect label="the JSON serialization XML format should be correct.">
+                <map xmlns="http://www.w3.org/2005/xpath-functions">
+                    <map key="root">
+                        <map key="field1">
+                            <string key="STRVALUE"/>
+                        </map>
+                    </map>
+                </map>
+            </x:expect>
+        </x:scenario>
+        <x:scenario label="If a document instance has a field with a value defined and no optional flag defined">
+            <x:context  mode="write-json">
+                <!--
+                    <root xmlns="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                        <field1>https://example.com</field1>
+                    </root>
+                -->
+                <assembly xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0/supermodel"
+                    name="root-assembly"
+                    key="root"
+                    gi="root"
+                    namespace="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                    <field collapsible="no"
+                        as-type="uri"
+                        name="field1"
+                        key="field1"
+                        gi="field1">
+                        <value as-type="uri" key="STRVALUE" in-json="string">https://example.com</value>
+                    </field>
+                </assembly>
+            </x:context>
+            <x:expect label="the JSON serialization XML format should be correct.">
+                <map xmlns="http://www.w3.org/2005/xpath-functions">
+                    <map key="root">
+                        <map key="field1">
+                            <string key="STRVALUE">https://example.com</string>
+                        </map>
+                    </map>
+                </map>
+            </x:expect>
+        </x:scenario>
+        <x:scenario label="If a document instance has its optional flag and a value defined">
+            <x:context  mode="write-json">
+                <!--
+                    <root xmlns="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                        <field1 optional-flag="value2">https://example.com</field1>
+                    </root>
+                -->
+                <assembly xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0/supermodel"
+                    name="root-assembly"
+                    key="root"
+                    gi="root"
+                    namespace="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                    <field collapsible="no"
+                        as-type="uri"
+                        name="field1"
+                        key="field1"
+                        gi="field1">
+                        <flag in-json="string"
+                            as-type="string"
+                            name="optional-flag"
+                            key="optional-flag"
+                            gi="optional-flag">value2</flag>
+                        <value as-type="uri" key="STRVALUE" in-json="string">https://example.com</value>
+                    </field>
+                </assembly>
+            </x:context>
+            <x:expect label="the JSON serialization XML format should be correct.">
+                <map xmlns="http://www.w3.org/2005/xpath-functions">
+                    <map key="root">
+                        <map key="field1">
+                            <string key="optional-flag">value2</string>
+                            <string key="STRVALUE">https://example.com</string>
+                        </map>
+                    </map>
+                </map>
+            </x:expect>
+        </x:scenario>
+        <x:scenario label="If a document instance has its optional flag defined and no value defined">
+            <x:context  mode="write-json">
+                <!--
+                    <root xmlns="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                        <field1 optional-flag="value2"/>
+                    </root>
+                -->
+                <assembly xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0/supermodel"
+                    name="root-assembly"
+                    key="root"
+                    gi="root"
+                    namespace="http://csrc.nist.gov/ns/metaschema/unit-tests">
+                    <field collapsible="no"
+                        as-type="uri"
+                        name="field1"
+                        key="field1"
+                        gi="field1">
+                        <flag in-json="string"
+                            as-type="string"
+                            name="optional-flag"
+                            key="optional-flag"
+                            gi="optional-flag">value2</flag>
+                        <value as-type="uri" key="STRVALUE" in-json="string"/>
+                    </field>
+                </assembly>
+            </x:context>
+            <x:expect label="the JSON serialization XML format should be correct.">
+                <map xmlns="http://www.w3.org/2005/xpath-functions">
+                    <map key="root">
+                        <map key="field1">
+                            <string key="optional-flag">value2</string>
+                            <string key="STRVALUE"/>
+                        </map>
+                    </map>
+                </map>
+            </x:expect>
+        </x:scenario>
+    </x:scenario>
+</x:description>
\ No newline at end of file