diff --git a/com.ibm.wala.core/src/com/ibm/wala/util/intset/SparseIntSet.java b/com.ibm.wala.core/src/com/ibm/wala/util/intset/SparseIntSet.java
index 761157120..082050e0c 100644
--- a/com.ibm.wala.core/src/com/ibm/wala/util/intset/SparseIntSet.java
+++ b/com.ibm.wala.core/src/com/ibm/wala/util/intset/SparseIntSet.java
@@ -349,6 +349,9 @@ public class SparseIntSet implements IntSet {
   }
 
   public IntSet intersection(IntSet that) {
+    if (that == null) {
+      throw new IllegalArgumentException("that == null");
+    }
     if (that instanceof SparseIntSet) {
       MutableSparseIntSet temp = MutableSparseIntSet.make(this);
       temp.intersectWith((SparseIntSet) that);