diff --git a/_test/options.txt b/_test/options.txt
index 99bc29c..433c49f 100644
--- a/_test/options.txt
+++ b/_test/options.txt
@@ -60,3 +60,11 @@
 //- - - - - - - - -//
 <h2 id="test--hey-sortclassfineclassshell-doesnt-matter">Test ## {#hey .sort,class=fine,class=shell} Doesn't matter</h2>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+
+6: class must be a string
+//- - - - - - - - -//
+# Test ## {class=0#.}
+//- - - - - - - - -//
+<h1 id="test--class0">Test ## {class=0#.}</h1>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
diff --git a/parser/attribute.go b/parser/attribute.go
index ea8c064..4c7cc24 100644
--- a/parser/attribute.go
+++ b/parser/attribute.go
@@ -129,6 +129,11 @@ func parseAttribute(reader text.Reader) (Attribute, bool) {
 	if !ok {
 		return Attribute{}, false
 	}
+	if bytes.Equal(name, attrNameClass) {
+		if _, ok = value.([]byte); !ok {
+			return Attribute{}, false
+		}
+	}
 	return Attribute{Name: name, Value: value}, true
 }