title: Pig: Failed to parse: mismatched input 'id' expecting set null
body: <p>I am using <a href="http://pig.apache.org/docs/r0.12.1/basic.html" rel="nofollow">Pig 0.12.1</a> and have the following Pig code:</p>
  
  <pre><code>C = LOAD '$file' USING myCustomLoader();
  D = FOREACH C GENERATE key#id;
  </code></pre>
  
  <p>I am loading a file using a custom loader. I then want to generate all of the IDs stored in <code>key</code>, a map.</p>
  
  <p>Why am I getting the following error message:</p>
  
  <pre><code>14/06/27 16:56:21 ERROR pig.PigServer: exception during parsing: Error during parsing.     &lt;line 3, column 28&gt;  mismatched input 'id' expecting set null
  Failed to parse: &lt;line 3, column 28&gt;  mismatched input 'id' expecting set null
  </code></pre>
  
  <p>Here is the full stacktrace:</p>
  
  <pre><code>14/06/27 16:56:21 ERROR pig.PigServer: exception during parsing: Error during parsing.     &lt;line 3, column 28&gt;  mismatched input 'id' expecting set null
  Failed to parse: &lt;line 3, column 28&gt;  mismatched input 'id' expecting set null
          at org.apache.pig.parser.QueryParserDriver.parse(QueryParserDriver.java:241)
          at org.apache.pig.parser.QueryParserDriver.parse(QueryParserDriver.java:179)
          at org.apache.pig.PigServer$Graph.parseQuery(PigServer.java:1676)
          at org.apache.pig.PigServer$Graph.registerQuery(PigServer.java:1623)
          at org.apache.pig.PigServer.registerQuery(PigServer.java:575)
          at org.apache.pig.tools.grunt.GruntParser.processPig(GruntParser.java:1093)
          at org.apache.pig.pigunit.pig.GruntParser.processPig(GruntParser.java:61)
          at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:501)
          at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:198)
          at org.apache.pig.pigunit.pig.PigServer.registerScript(PigServer.java:56)
          at org.apache.pig.pigunit.PigTest.registerScript(PigTest.java:170)
          at org.apache.pig.pigunit.PigTest.assertOutput(PigTest.java:249)
          at com.testpig.PigTest.testPig(PigTest.java:159)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
          at java.lang.reflect.Method.invoke(Method.java:601)
          at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
          at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
          at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
          at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
          at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
          at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
          at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
          at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
          at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
          at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
          at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
          at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
          at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
          at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
          at org.apache.maven.surefire.junit4.JUnit4TestSet.execute(JUnit4TestSet.java:53)
          at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:123)
          at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:104)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
          at java.lang.reflect.Method.invoke(Method.java:601)
          at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:164)
          at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:110)
          at org.apache.maven.surefire.booter.SurefireStarter.invokeProvider(SurefireStarter.java:172)
          at org.apache.maven.surefire.booter.SurefireStarter.runSuitesInProcessWhenForked(SurefireStarter.java:104)
          at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:70)
  14/06/27 16:56:21 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
  14/06/27 16:56:21 INFO compress.CodecPool: Got brand-new compressor
  </code></pre>