diff --git a/lib/forceCluster.js b/lib/forceCluster.js
index 2c9f1ff..64ebbda 100644
--- a/lib/forceCluster.js
+++ b/lib/forceCluster.js
@@ -25,7 +25,7 @@ function cluster (centers) {
       x = d.x - c.x,
       y = d.y - c.y,
       l = Math.sqrt(x * x + y * y),
-      r = d.radius + (c.radius || 0);
+      r = d.radius || 1 + (c.radius || 0);
 
       if (l && l != r) {
         l = (l - r) / l * alpha;
@@ -96,4 +96,4 @@ function cluster (centers) {
 
 }
 
-export default cluster;
\ No newline at end of file
+export default cluster;