diff --git a/app/assets/javascripts/discourse/lib/to-markdown.js.es6 b/app/assets/javascripts/discourse/lib/to-markdown.js.es6
index 8f81536f22..1d2f2cbe25 100644
--- a/app/assets/javascripts/discourse/lib/to-markdown.js.es6
+++ b/app/assets/javascripts/discourse/lib/to-markdown.js.es6
@@ -234,9 +234,20 @@ class Tag {
   static table() {
     return class extends Tag.block("table") {
       decorate(text) {
-        text = super.decorate(text);
-        const splitterRow = text.split("|\n")[0].match(/\|/g).map(() => "| --- ").join("") + "|\n";
-        text = text.replace("|\n", "|\n" + splitterRow).replace(/\|\n{2,}\|/g, "|\n|");
+        text = super.decorate(text).replace(/\|\n{2,}\|/g, "|\n|");
+        const rows = text.trim().split("\n");
+        const pipes = rows[0].match(/\|/g);
+        const isValid = rows.length > 1 &&
+                        pipes.length > 2 &&
+                        rows.reduce((a, c) => a && c.match(/\|/g).length <= pipes.length);
+
+        if (!isValid) {
+          throw "Unsupported table format for Markdown conversion";
+        }
+
+        const splitterRow = pipes.slice(1).map(() => "| --- ").join("") + "|\n";
+        text = text.replace("|\n", "|\n" + splitterRow);
+
         return text;
       }
     };
diff --git a/test/javascripts/lib/to-markdown-test.js.es6 b/test/javascripts/lib/to-markdown-test.js.es6
index 4ebb94ec9a..0a33d642b9 100644
--- a/test/javascripts/lib/to-markdown-test.js.es6
+++ b/test/javascripts/lib/to-markdown-test.js.es6
@@ -109,7 +109,7 @@ QUnit.test("converts table tags", assert => {
 });
 
 QUnit.test("returns empty string if table format not supported", assert => {
-  const html = `<table>
+  let html = `<table>
     <thead> <tr><th>Headi\n\nng 1</th><th>Head 2</th></tr> </thead>
       <tbody>
         <tr><td>Lorem</td><td>ipsum</td></tr>
@@ -117,6 +117,18 @@ QUnit.test("returns empty string if table format not supported", assert => {
 </table>
   `;
   assert.equal(toMarkdown(html), "");
+
+  html = `<table>
+    <thead> <tr><th>Heading 1</th></tr> </thead>
+      <tbody>
+        <tr><td>Lorem</td></tr>
+        <tr><td><i>sit amet</i></td></tr></tbody>
+</table>
+  `;
+  assert.equal(toMarkdown(html), "");
+
+  html = `<table><tr><td>Lorem</td><td><i>sit amet</i></td></tr></table>`;
+  assert.equal(toMarkdown(html), "");
 });
 
 QUnit.test("converts img tag", assert => {