diff --git a/lib/type/matrix/DenseFormat.js b/lib/type/matrix/DenseFormat.js
index 5a7727d26..6043063db 100644
--- a/lib/type/matrix/DenseFormat.js
+++ b/lib/type/matrix/DenseFormat.js
@@ -274,7 +274,7 @@ module.exports = function (math) {
 
     // calculate the size of the submatrix, and convert it into an Array if needed
     var sSize;
-    if (submatrix instanceof DenseFormat) {
+    if (submatrix instanceof math.type.Matrix) {
       sSize = submatrix.size();
       submatrix = submatrix.valueOf();
     }