There was a bug in the internal IntIntHashMap.

h2/src/main/org/h2/util/IntIntHashMap.java

@@ -42,6 +42,7 @@ public class IntIntHashMap extends HashBase {
         if (key == 0) {
             zeroKey = true;
             zeroValue = value;
+            return;
         }
         try {
             checkSizePut();

h2/src/test/org/h2/test/unit/TestIntIntHashMap.java

@@ -28,6 +28,13 @@ public class TestIntIntHashMap extends TestBase {
     }
 
     public void test() {
+        IntIntHashMap map = new IntIntHashMap();
+        map.put(1, 1);
+        map.put(1, 2);
+        assertEquals(1, map.size());
+        map.put(0, 1);
+        map.put(0, 2);
+        assertEquals(2, map.size());
         rand.setSeed(10);
         test(true);
         test(false);