diff --git a/app/assets/javascripts/discourse/dialects/bbcode_dialect.js b/app/assets/javascripts/discourse/dialects/bbcode_dialect.js
index 0e154527a74..5942857ede8 100644
--- a/app/assets/javascripts/discourse/dialects/bbcode_dialect.js
+++ b/app/assets/javascripts/discourse/dialects/bbcode_dialect.js
@@ -90,7 +90,7 @@ replaceBBCodeParamsRaw("email", function(param, contents) {
 });
 
 replaceBBCodeParams("size", function(param, contents) {
-  return ['span', {'class': "bbcode-size-" + param}].concat(contents);
+  return ['span', {'class': "bbcode-size-" + (parseInt(param) || 1)}].concat(contents);
 });
 
 // Handles `[code] ... [/code]` blocks
diff --git a/test/javascripts/components/bbcode_test.js b/test/javascripts/components/bbcode_test.js
index 0c06638576f..64ef9ec5ff5 100644
--- a/test/javascripts/components/bbcode_test.js
+++ b/test/javascripts/components/bbcode_test.js
@@ -37,13 +37,21 @@ test('lists', function() {
 });
 
 test('tags with arguments', function() {
-  format("[size=35]BIG [b]whoop[/b][/size]", "<span class=\"bbcode-size-35\">BIG <span class=\"bbcode-b\">whoop</span></span>", "supports [size=]");
   format("[url=http://bettercallsaul.com]better call![/url]", "<a href=\"http://bettercallsaul.com\">better call!</a>", "supports [url] with a title");
   format("[email=eviltrout@mailinator.com]evil trout[/email]", "<a href=\"mailto:eviltrout@mailinator.com\">evil trout</a>", "supports [email] with a title");
   format("[u][i]abc[/i][/u]", "<span class=\"bbcode-u\"><span class=\"bbcode-i\">abc</span></span>", "can nest tags");
   format("[b]first[/b] [b]second[/b]", "<span class=\"bbcode-b\">first</span> <span class=\"bbcode-b\">second</span>", "can bold two things on the same line");
 });
 
+test("size tags", function() {
+  format("[size=35]BIG [b]whoop[/b][/size]",
+         "<span class=\"bbcode-size-35\">BIG <span class=\"bbcode-b\">whoop</span></span>",
+         "supports [size=]");
+  format("[size=asdf]regular[/size]",
+         "<span class=\"bbcode-size-1\">regular</span>",
+         "it only supports numbers in bbcode");
+});
+
 test("quotes", function() {
 
   var post = Discourse.Post.create({