diff --git a/lib/extramath.js b/lib/extramath.js
index 2a7254f..594f85d 100644
--- a/lib/extramath.js
+++ b/lib/extramath.js
@@ -73,7 +73,7 @@ function cartesianProduct(arr) {
 function clusteredCellsDensity(numbers, size, minDensity) {
     if (!numbers || !numbers.length) return false;
     if (typeof size !== 'number' || size <= 0) size = 4; // default grid size = 4
-    if (typeof minDensity === 'undefined') minDensity = 0.6;
+    if (typeof minDensity !== 'number' || isNaN(minDensity)) minDensity = 0.6;
 
     var maxIndex = size * size; // because it's 1-based (1..size*size)