Index: src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java =================================================================== --- src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java (revision 1226899) +++ src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java (working copy) @@ -2268,7 +2268,14 @@ s = r.getScanner(scan); } if (r.getCoprocessorHost() != null) { - s = r.getCoprocessorHost().postScannerOpen(scan, s); + RegionScanner savedScanner = r.getCoprocessorHost(). + postScannerOpen(scan, s); + if (savedScanner == null) { + LOG.warn("PostScannerOpen impl returning null. " + + "Check the RegionObserver implementation."); + } else { + s = savedScanner; + } } return addScanner(s); } catch (Throwable t) {