diff --git a/src/render/index.js b/src/render/index.js
index 7ab1b20..3ae5221 100644
--- a/src/render/index.js
+++ b/src/render/index.js
@@ -64,6 +64,24 @@ const getFormat = (input) => {
     }
 };
 
+/**
+ * Validate MathML string.
+ *
+ * @param {string} math - The input MathML string to validate.
+ * @param {Promise} conversionPromise - The MathJax conversion function to use for validation.
+ * @returns {Promise<node: string}>} The MathJax conversion promise
+ * @throws {Error} Throws an error if input string is not valid MathML.
+ */
+const validateMathML = async (math, conversionPromise) => {
+    const p = conversionPromise(math);
+    try {
+        await p;
+    } catch(err) {
+        throw new Error(`Invalid MathML: ${err.message}`);
+    }
+    return p;
+}
+
 /**
  * Typeset math.
  *
@@ -85,7 +103,10 @@ const typeset = async (data) => {
                 throw new Error(`Supported output formats for ${data.format} input are: MathML, SVG`);
             case 'MathML':
                 if (data.svg) {
-                    return MathJax.mathml2svgPromise(data.math);
+                    return await validateMathML(data.math, MathJax.mathml2svgPromise);
+                }
+                if (data.mml) {
+                    return await validateMathML(data.math, MathJax.mathml2mmlPromise);
                 }
                 throw new Error(`Supported output formats for ${data.format} input are: SVG`);
             default:
@@ -116,11 +137,6 @@ const typeset = async (data) => {
  * @returns {Promise<Output>}
  */
 exports.render = async (event) => {
-    if (event.input === 'mathml' && event.output === 'mathml') {
-        // No-op conversion MathML-to-MathML.
-        return { contentType: RESPONSE_TYPES.mathml, data: event.source };
-    }
-
     const format = getFormat(event);
     const math = event.source;
     switch (event.output) {
diff --git a/test/app.js b/test/app.js
index 1a52629..62ee31e 100644
--- a/test/app.js
+++ b/test/app.js
@@ -90,6 +90,26 @@ describe('GET /render', function () {
             .expect(400, { message: 'Invalid output: INVALID' })
             .expect('Content-Type', /^application\/json(?:;|$)/);
     });
+
+    it('should return "400 Bad Request" with invalid MathML source (xml validation error)', function () {
+        const search = { input: 'mathml', output: 'mathml', source: 'x^2' };
+        const url = `/render?${querystring.stringify(search)}`;
+
+        return testApp
+            .get(url)
+            .expect(400, { message: 'Invalid MathML: MathML must be formed by a <math> element, not <#text>' })
+            .expect('Content-Type', /^application\/json(?:;|$)/);
+    });
+
+    it('should return "400 Bad Request" with invalid MathML source (MathML validation error)', function () {
+        const search = { input: 'mathml', output: 'mathml', source: '<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">x^2</math>' };
+        const url = `/render?${querystring.stringify(search)}`;
+
+        return testApp
+            .get(url)
+            .expect(400, { message: 'Invalid MathML: Unexpected text node "x^2"' })
+            .expect('Content-Type', /^application\/json(?:;|$)/);
+    });
 });
 
 describe('POST /render', function () {
@@ -157,4 +177,26 @@ describe('POST /render', function () {
             .expect(400, { message: 'Invalid output: INVALID' })
             .expect('Content-Type', /^application\/json(?:;|$)/);
     });
+
+    it('should return "400 Bad Request" with invalid MathML source (xml validation error)', function () {
+        const body = { input: 'mathml', output: 'mathml', source: 'x^2' };
+
+        return testApp
+            .post('/render')
+            .set('Content-Type', 'application/json')
+            .send(body)
+            .expect(400, { message: 'Invalid MathML: MathML must be formed by a <math> element, not <#text>' })
+            .expect('Content-Type', /^application\/json(?:;|$)/);
+    });
+
+    it('should return "400 Bad Request" with invalid MathML source (MathML validation error)', function () {
+        const body = { input: 'mathml', output: 'mathml', source: '<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">x^2</math>' };
+
+        return testApp
+            .post('/render')
+            .set('Content-Type', 'application/json')
+            .send(body)
+            .expect(400, { message: 'Invalid MathML: Unexpected text node "x^2"' })
+            .expect('Content-Type', /^application\/json(?:;|$)/);
+    });
 });
diff --git a/test/render/index.js b/test/render/index.js
index e0616e4..fdc280c 100644
--- a/test/render/index.js
+++ b/test/render/index.js
@@ -90,7 +90,7 @@ describe('index#render', function () {
         }
     });
 
-    it('should throw with invalid source', async function () {
+    it('should throw with invalid xml source', async function () {
         const event = { input: 'mathml', output: 'svg', source: 'x^2' };
 
         try {
@@ -99,7 +99,21 @@ describe('index#render', function () {
         } catch (err) {
             expect(err).to.be.an.instanceOf(Error)
                 .that.has.property('message')
-                    .that.contains('MathML must be formed by a <math> element, not <#text>')
+                    .that.contains('Invalid MathML: MathML must be formed by a <math> element, not <#text>')
+                    .and.that.does.not.contain('\n');
+        }
+    });
+
+    it('should throw with valid xml but invalid MathML source', async function () {
+        const event = { input: 'mathml', output: 'svg', source: '<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">x^2</math>' };
+
+        try {
+            await render(event);
+            expect.fail('Did not throw');
+        } catch (err) {
+            expect(err).to.be.an.instanceOf(Error)
+                .that.has.property('message')
+                    .that.contains('Invalid MathML: Unexpected text node "x^2"')
                     .and.that.does.not.contain('\n');
         }
     });