namenode generates infinite stream of null pointers

My namenode is generating a constant stream of NullPointerExceptions. The log looks like:

2007-01-17 19:51:27,461 INFO org.apache.hadoop.ipc.Server: IPC Server handler 3
on 50000 call error: java.io.IOException: java.lang.NullPointerException
java.io.IOException: java.lang.NullPointerException
at org.apache.hadoop.dfs.FSNamesystem.addStoredBlock(FSNamesystem.java:1621)
at org.apache.hadoop.dfs.FSNamesystem.processReport(FSNamesystem.java:1563)
at org.apache.hadoop.dfs.NameNode.blockReport(NameNode.java:573)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:589)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:337)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:538)