diff --git a/projects/Chart/19/org/jfree/chart/plot/CategoryPlot.java b/projects/Chart/19/org/jfree/chart/plot/CategoryPlot.java
index cf3f52f..ec31907 100644
--- a/projects/Chart/19/org/jfree/chart/plot/CategoryPlot.java
+++ b/projects/Chart/19/org/jfree/chart/plot/CategoryPlot.java
@@ -695,6 +695,9 @@ public void setDomainAxes(CategoryAxis[] axes) {
      * @since 1.0.3
      */
     public int getDomainAxisIndex(CategoryAxis axis) {
+        if (axis == null) {
+            throw new IllegalArgumentException("Null 'axis' argument.");
+        }
         return this.domainAxes.indexOf(axis);
     }
     
@@ -970,6 +973,9 @@ public void setRangeAxes(ValueAxis[] axes) {
      * @since 1.0.7
      */
     public int getRangeAxisIndex(ValueAxis axis) {
+        if (axis == null) {
+            throw new IllegalArgumentException("Null 'axis' argument.");
+        }
         int result = this.rangeAxes.indexOf(axis);
         if (result < 0) { // try the parent plot
             Plot parent = getParent();