diff --git a/CHANGES.txt b/CHANGES.txt
index f19be4e71cc..37a9bfebcaf 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -185,6 +185,8 @@ Release 0.91.0 - Unreleased
    HBASE-4148  HFileOutputFormat doesn't fill in TIMERANGE_KEY metadata (Jonathan Hsieh)
    HBASE-4003  Cleanup Calls Conservatively On Timeout (Karthick)
    HBASE-3857  Fix TestHFileBlock.testBlockHeapSize test failure (Mikhail)
+   HBASE-4150  Potentially too many connections may be opened if ThreadLocalPool
+               or RoundRobinPool is used
 
   IMPROVEMENTS
    HBASE-3290  Max Compaction Size (Nicolas Spiegelberg via Stack)  
diff --git a/src/main/java/org/apache/hadoop/hbase/util/PoolMap.java b/src/main/java/org/apache/hadoop/hbase/util/PoolMap.java
index fba06b76b54..80ea4204bb7 100644
--- a/src/main/java/org/apache/hadoop/hbase/util/PoolMap.java
+++ b/src/main/java/org/apache/hadoop/hbase/util/PoolMap.java
@@ -397,7 +397,8 @@ public class PoolMap<K, V> implements Map<K, V> {
           poolSizes.put(this, poolSize = new AtomicInteger(0));
         }
         if (poolSize.intValue() >= maxSize) {
-          return null;
+          throw new IllegalStateException("poolSize ("+poolSize.intValue()+
+              ") has reached " + maxSize);
         }
         poolSize.incrementAndGet();
       }