diff --git a/hadoop-mapreduce-project/CHANGES.txt b/hadoop-mapreduce-project/CHANGES.txt
index 9e6f6dff42f..d293b52b3f7 100644
--- a/hadoop-mapreduce-project/CHANGES.txt
+++ b/hadoop-mapreduce-project/CHANGES.txt
@@ -203,6 +203,10 @@ Release 0.23.2 - UNRELEASED
     MAPREDUCE-3816. capacity scheduler web ui bar graphs for used capacity wrong
     (tgraves via bobby)
 
+    MAPREDUCE-3930. Fixed an NPE while accessing the AM page/webservice for a 
+    task attempt without an assigned container. (Robert Joseph Evans via
+    sseth)
+
 Release 0.23.1 - 2012-02-17
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/util/ConverterUtils.java b/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/util/ConverterUtils.java
index fdebd3925b8..21fe2d9874c 100644
--- a/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/util/ConverterUtils.java
+++ b/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/util/ConverterUtils.java
@@ -142,7 +142,7 @@ private static ApplicationId toApplicationId(
   }
 
   public static String toString(ContainerId cId) {
-    return cId.toString();
+    return cId == null ? null : cId.toString();
   }
 
   public static NodeId toNodeId(String nodeIdStr) {
diff --git a/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/test/java/org/apache/hadoop/yarn/util/TestConverterUtils.java b/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/test/java/org/apache/hadoop/yarn/util/TestConverterUtils.java
index 0e2eb14a40a..7924124bf40 100644
--- a/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/test/java/org/apache/hadoop/yarn/util/TestConverterUtils.java
+++ b/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/test/java/org/apache/hadoop/yarn/util/TestConverterUtils.java
@@ -22,6 +22,7 @@
 import java.net.URISyntaxException;
 
 import org.apache.hadoop.fs.Path;
+import org.apache.hadoop.yarn.api.records.ContainerId;
 import org.apache.hadoop.yarn.api.records.URL;
 import org.junit.Test;
 
@@ -35,4 +36,17 @@ public void testConvertUrlWithNoPort() throws URISyntaxException {
     assertEquals(expectedPath, actualPath);
   }
 
+  @Test
+  public void testContainerId() throws URISyntaxException {
+    ContainerId id = BuilderUtils.newContainerId(0, 0, 0, 0);
+    String cid = ConverterUtils.toString(id);
+    assertEquals("container_0_0000_00_000000", cid);
+    ContainerId gen = ConverterUtils.toContainerId(cid);
+    assertEquals(gen, id);
+  }
+
+  @Test
+  public void testContainerIdNull() throws URISyntaxException {
+    assertNull(ConverterUtils.toString((ContainerId)null));
+  }  
 }