diff --git a/tool/formula/src/main/java/org/openscience/cdk/formula/MolecularFormulaRange.java b/tool/formula/src/main/java/org/openscience/cdk/formula/MolecularFormulaRange.java
index 3dc2d5ea773..d50d3651213 100644
--- a/tool/formula/src/main/java/org/openscience/cdk/formula/MolecularFormulaRange.java
+++ b/tool/formula/src/main/java/org/openscience/cdk/formula/MolecularFormulaRange.java
@@ -64,6 +64,10 @@ public MolecularFormulaRange() {
      *
      */
     public void addIsotope(IIsotope isotope, int countMin, int countMax) {
+        
+        if (isotope == null) 
+            throw new IllegalArgumentException("Isotope must not be null");
+        
         boolean flag = false;
         for (Iterator<IIsotope> it = isotopes().iterator(); it.hasNext();) {
             IIsotope thisIsotope = it.next();
diff --git a/tool/formula/src/test/java/org/openscience/cdk/formula/MolecularFormulaRangeTest.java b/tool/formula/src/test/java/org/openscience/cdk/formula/MolecularFormulaRangeTest.java
index 71669a3977f..b48c6253ab4 100644
--- a/tool/formula/src/test/java/org/openscience/cdk/formula/MolecularFormulaRangeTest.java
+++ b/tool/formula/src/test/java/org/openscience/cdk/formula/MolecularFormulaRangeTest.java
@@ -375,4 +375,17 @@ public void testClone_Isotopes() throws Exception {
         Assert.assertEquals(8, ((MolecularFormulaRange) clone).getIsotopeCountMax(flu));
         Assert.assertEquals(10, ((MolecularFormulaRange) clone).getIsotopeCountMax(h1));
     }
+    
+    /**
+     * Test what happens when null isotope is added to MF range.
+     */
+    @Test(expected=IllegalArgumentException.class)
+    public void testNull() throws Exception {
+        MolecularFormulaRange mfRange = new MolecularFormulaRange();
+        IIsotope carb = builder.newInstance(IIsotope.class, "C");
+        IIsotope nul = null;
+        mfRange.addIsotope(carb, 2, 5);
+        mfRange.addIsotope(nul, 3, 7);
+    }
+    
 }