diff --git a/src/TemplateMarkInterpreter.ts b/src/TemplateMarkInterpreter.ts
index 4654be5..49a89b5 100644
--- a/src/TemplateMarkInterpreter.ts
+++ b/src/TemplateMarkInterpreter.ts
@@ -575,13 +575,61 @@ export class TemplateMarkInterpreter {
         const serializer = new Serializer(factory, modelManager);
         try {
             serializer.fromJSON(templateMark);
-            return templateMark;
         }
         catch (err) {
             throw new Error(`Generated invalid agreement: ${err}: ${JSON.stringify(templateMark, null, 2)}`);
         }
+
+        const errors: Array<{ propertyName: string, message: string }> = [];
+        const templateClass = this.templateClass;
+        const guardBlockPaths: Map<string, string> = new Map();
+
+        traverse(templateMark).forEach(function (node: any) {
+            if (!node || typeof node !== 'object' || !node.$class) return;
+
+            const currentPath = this.path.join('/');
+            if (OPTIONAL_DEFINITION_RE.test(node.$class) ||
+                CONDITIONAL_DEFINITION_RE.test(node.$class) ||
+                WITH_DEFINITION_RE.test(node.$class)) {
+                guardBlockPaths.set(node.name, currentPath);
+            }
+            if (VARIABLE_DEFINITION_RE.test(node.$class) ||
+                ENUM_VARIABLE_DEFINITION_RE.test(node.$class) ||
+                FORMATTED_VARIABLE_DEFINITION_RE.test(node.$class)) {
+                const propName = node.name;
+                if (propName && propName !== 'this') {
+                    try {
+                        const property = templateClass.getProperty(propName);
+                        if (property && property.isOptional()) {
+                            const guardPath = guardBlockPaths.get(propName);
+                            const isGuarded = guardPath !== undefined &&
+                                (currentPath === guardPath || currentPath.startsWith(guardPath + '/'));
+                            if (!isGuarded) {
+                                errors.push({
+                                    propertyName: propName,
+                                    message: `Optional property '${propName}' is used without a guard. Wrap it in {{#optional ${propName}}}...{{/optional}} or {{#if ${propName}}}...{{/if}}.`
+                                });
+                            }
+                        }
+                    } catch {
+                        // Property not found at root level, might be nested - skip
+                    }
+                }
+            }
+        });
+
+        if (errors.length > 0) {
+            const errorMessage = `Optional properties used without guards: ${errors.map(e => e.propertyName).join(', ')}`;
+            const error = new Error(errorMessage);
+            (error as any).errors = errors;
+            throw error;
+        }
+
+        return templateMark;
     }
 
+
+
     /**
      * Compiles the code nodes containing TS to code nodes containing JS.
      * @param {*} templateMark the TemplateMark JSON object
diff --git a/test/__snapshots__/TemplateMarkInterpreter.test.ts.snap b/test/__snapshots__/TemplateMarkInterpreter.test.ts.snap
index c439263..676df9b 100644
--- a/test/__snapshots__/TemplateMarkInterpreter.test.ts.snap
+++ b/test/__snapshots__/TemplateMarkInterpreter.test.ts.snap
@@ -82,26 +82,9 @@ exports[`templatemark interpreter should fail to generate formula-no-method 1`]
 ]
 `;
 
-exports[`templatemark interpreter should fail to generate formula-optional-noguard 1`] = `
-[
-  {
-    "code": " return message.length ",
-    "errors": [
-      {
-        "category": 1,
-        "character": 10,
-        "code": 18048,
-        "id": "err-18048-475-7",
-        "length": 7,
-        "line": 82,
-        "renderedMessage": "'message' is possibly 'undefined'.",
-        "start": 1793,
-      },
-    ],
-    "nodeId": "formula_a3e0e90a2ad3601e1b208a0581e1ce6dfc5aab302d20cb2a0488d78e236096f7",
-  },
-]
-`;
+exports[`templatemark interpreter should fail to generate formula-optional-noguard 1`] = `[Error: Optional properties used without guards: message]`;
+
+exports[`templatemark interpreter should fail to generate optional-noguard 1`] = `[Error: Optional properties used without guards: nickname]`;
 
 exports[`templatemark interpreter should fail to generate template-missing-field 1`] = `[TemplateException: Unknown property: missing File text/grammar.tem.md line -1 column -1]`;
 
diff --git a/test/templates/bad/optional-noguard/data.json b/test/templates/bad/optional-noguard/data.json
new file mode 100644
index 0000000..f54f4c4
--- /dev/null
+++ b/test/templates/bad/optional-noguard/data.json
@@ -0,0 +1,4 @@
+{
+    "$class": "test@1.0.0.TemplateData",
+    "name": "World"
+}
diff --git a/test/templates/bad/optional-noguard/model.cto b/test/templates/bad/optional-noguard/model.cto
new file mode 100644
index 0000000..a737509
--- /dev/null
+++ b/test/templates/bad/optional-noguard/model.cto
@@ -0,0 +1,7 @@
+namespace test@1.0.0
+
+@template
+concept TemplateData {
+    o String name
+    o String nickname optional
+}
diff --git a/test/templates/bad/optional-noguard/template.md b/test/templates/bad/optional-noguard/template.md
new file mode 100644
index 0000000..1ee156f
--- /dev/null
+++ b/test/templates/bad/optional-noguard/template.md
@@ -0,0 +1 @@
+Hello {{name}}! Your nickname is {{nickname}}.
diff --git a/test/templates/good/playground/model.cto b/test/templates/good/playground/model.cto
index 13acfb7..33c2059 100644
--- a/test/templates/good/playground/model.cto
+++ b/test/templates/good/playground/model.cto
@@ -23,7 +23,7 @@ concept Order {
 concept TemplateData {
     o String name
     o Address address
-    o Integer age optional
+    o Integer age
     o MonetaryAmount salary
     o String[] favoriteColors
     o Order order