diff --git a/SimpleActivityStack/src/solution/SimpleActivityStack.java b/SimpleActivityStack/src/solution/SimpleActivityStack.java
index a183c0c..1d2a223 100644
--- a/SimpleActivityStack/src/solution/SimpleActivityStack.java
+++ b/SimpleActivityStack/src/solution/SimpleActivityStack.java
@@ -6,6 +6,9 @@ public class SimpleActivityStack extends SimpleStack<SimpleActivity> {
 
 	@Override
 	public int push(SimpleActivity activity) {
+		if (activity == null) {
+			return 0;
+		}
 		if (size() > 0) {
 			super.get(size() - 1).passivate();
 		}