diff --git a/java/src/org/broadinstitute/sting/gatk/AbstractGenomeAnalysisEngine.java b/java/src/org/broadinstitute/sting/gatk/AbstractGenomeAnalysisEngine.java
index 752fdaa3b..dc0ea8ab6 100755
--- a/java/src/org/broadinstitute/sting/gatk/AbstractGenomeAnalysisEngine.java
+++ b/java/src/org/broadinstitute/sting/gatk/AbstractGenomeAnalysisEngine.java
@@ -494,14 +494,9 @@ public abstract class AbstractGenomeAnalysisEngine {
         return method;
     }
 
-    protected void validateSuppliedReads() {
-    }
-
-    protected void validateSuppliedReference() {
-    }
-
-    protected void validateSuppliedReferenceOrderedData(List<RMDTrack> rods) {
-    }
+    protected abstract void validateSuppliedReads();
+    protected abstract void validateSuppliedReference();
+    protected abstract void validateSuppliedReferenceOrderedData(List<RMDTrack> rods);
 
     /**
      * Now that all files are open, validate the sequence dictionaries of the reads vs. the reference vrs the reference ordered data (if available).
diff --git a/java/src/org/broadinstitute/sting/gatk/GenomeAnalysisEngine.java b/java/src/org/broadinstitute/sting/gatk/GenomeAnalysisEngine.java
index 2a006e7de..914cbcc05 100755
--- a/java/src/org/broadinstitute/sting/gatk/GenomeAnalysisEngine.java
+++ b/java/src/org/broadinstitute/sting/gatk/GenomeAnalysisEngine.java
@@ -96,7 +96,9 @@ public class GenomeAnalysisEngine extends AbstractGenomeAnalysisEngine {
         // create the output streams                     "
         initializeOutputStreams(microScheduler.getOutputTracker());
 
+        // initialize and validate the interval list                
         initializeIntervals();
+        validateSuppliedIntervals();
 
         ShardStrategy shardStrategy = getShardStrategy(microScheduler.getReference());
 
@@ -254,6 +256,15 @@ public class GenomeAnalysisEngine extends AbstractGenomeAnalysisEngine {
         }
     }
 
+    protected void validateSuppliedIntervals() {
+        // Only read walkers support '-L unmapped' intervals.  Trap and validate any other instances of -L unmapped.
+        if(!(walker instanceof ReadWalker)) {
+            GenomeLocSortedSet intervals = getIntervals();
+            if(intervals != null && getIntervals().contains(GenomeLoc.UNMAPPED))
+                throw new ArgumentException("Interval list specifies unmapped region.  Only read walkers may include the unmapped region.");
+        }
+    }
+
     /**
      * Get the sharding strategy given a driving data source.
      *
diff --git a/java/test/org/broadinstitute/sting/utils/GenomeLocUnitTest.java b/java/test/org/broadinstitute/sting/utils/GenomeLocUnitTest.java
index 60062583f..29c085b70 100644
--- a/java/test/org/broadinstitute/sting/utils/GenomeLocUnitTest.java
+++ b/java/test/org/broadinstitute/sting/utils/GenomeLocUnitTest.java
@@ -5,6 +5,8 @@ package org.broadinstitute.sting.utils;
 // the imports for unit testing.
 
 
+import org.broadinstitute.sting.utils.interval.IntervalMergingRule;
+import org.broadinstitute.sting.utils.interval.IntervalUtils;
 import org.testng.Assert;
 import org.testng.annotations.BeforeClass;
 import org.testng.annotations.Test;
@@ -13,6 +15,9 @@ import org.broadinstitute.sting.utils.fasta.CachingIndexedFastaSequenceFile;
 
 import java.io.File;
 import java.io.FileNotFoundException;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 
 import net.sf.picard.reference.ReferenceSequenceFile;
 import net.sf.picard.reference.IndexedFastaSequenceFile;
@@ -83,5 +88,66 @@ public class GenomeLocUnitTest extends BaseTest {
     }
 
 
+    @Test
+    public void testUnmappedSort() {
+        GenomeLoc chr1 = genomeLocParser.createGenomeLoc("chr1",1,10000000);
+        GenomeLoc chr2 = genomeLocParser.createGenomeLoc("chr2",1,10000000);
+        GenomeLoc unmapped = GenomeLoc.UNMAPPED;
 
+        List<GenomeLoc> unmappedOnly = Arrays.asList(unmapped);
+        Collections.sort(unmappedOnly);
+        Assert.assertEquals(unmappedOnly.size(),1,"Wrong number of elements in unmapped-only list.");
+        Assert.assertEquals(unmappedOnly.get(0),unmapped,"List sorted in wrong order");
+
+        List<GenomeLoc> chr1Presorted = Arrays.asList(chr1,unmapped);
+        Collections.sort(chr1Presorted);
+        Assert.assertEquals(chr1Presorted.size(),2,"Wrong number of elements in chr1,unmapped list.");
+        Assert.assertEquals(chr1Presorted,Arrays.asList(chr1,unmapped),"List sorted in wrong order");
+
+        List<GenomeLoc> chr1Inverted = Arrays.asList(unmapped,chr1);
+        Collections.sort(chr1Inverted);
+        Assert.assertEquals(chr1Inverted.size(),2,"Wrong number of elements in chr1,unmapped list.");
+        Assert.assertEquals(chr1Inverted,Arrays.asList(chr1,unmapped),"List sorted in wrong order");
+
+        List<GenomeLoc> chr1and2Presorted = Arrays.asList(chr1,chr2,unmapped);
+        Collections.sort(chr1and2Presorted);
+        Assert.assertEquals(chr1and2Presorted.size(),3,"Wrong number of elements in chr1,chr2,unmapped list.");
+        Assert.assertEquals(chr1and2Presorted,Arrays.asList(chr1,chr2,unmapped),"List sorted in wrong order");
+
+        List<GenomeLoc> chr1and2UnmappedInFront = Arrays.asList(unmapped,chr1,chr2);
+        Collections.sort(chr1and2UnmappedInFront);
+        Assert.assertEquals(chr1and2UnmappedInFront.size(),3,"Wrong number of elements in unmapped,chr1,chr2 list.");
+        Assert.assertEquals(chr1and2UnmappedInFront,Arrays.asList(chr1,chr2,unmapped),"List sorted in wrong order");
+
+        List<GenomeLoc> chr1and2UnmappedSandwiched = Arrays.asList(chr1,unmapped,chr2);
+        Collections.sort(chr1and2UnmappedSandwiched);
+        Assert.assertEquals(chr1and2UnmappedSandwiched.size(),3,"Wrong number of elements in chr1,unmapped,chr2 list.");
+        Assert.assertEquals(chr1and2UnmappedSandwiched,Arrays.asList(chr1,chr2,unmapped),"List sorted in wrong order");
+    }
+
+    @Test
+    public void testUnmappedMerge() {
+        GenomeLoc chr1 = genomeLocParser.createGenomeLoc("chr1",1,10000000);
+        GenomeLoc unmapped = GenomeLoc.UNMAPPED;
+
+        List<GenomeLoc> oneUnmappedOnly = Arrays.asList(unmapped);
+        oneUnmappedOnly = IntervalUtils.sortAndMergeIntervals(genomeLocParser,oneUnmappedOnly, IntervalMergingRule.OVERLAPPING_ONLY).toList();
+        Assert.assertEquals(oneUnmappedOnly.size(),1,"Wrong number of elements in list.");
+        Assert.assertEquals(oneUnmappedOnly.get(0),unmapped,"List sorted in wrong order");
+
+        List<GenomeLoc> twoUnmapped = Arrays.asList(unmapped,unmapped);
+        twoUnmapped = IntervalUtils.sortAndMergeIntervals(genomeLocParser,twoUnmapped,IntervalMergingRule.OVERLAPPING_ONLY).toList();
+        Assert.assertEquals(twoUnmapped.size(),1,"Wrong number of elements in list.");
+        Assert.assertEquals(twoUnmapped.get(0),unmapped,"List sorted in wrong order");
+
+        List<GenomeLoc> twoUnmappedAtEnd = Arrays.asList(chr1,unmapped,unmapped);
+        twoUnmappedAtEnd = IntervalUtils.sortAndMergeIntervals(genomeLocParser,twoUnmappedAtEnd,IntervalMergingRule.OVERLAPPING_ONLY).toList();
+        Assert.assertEquals(twoUnmappedAtEnd.size(),2,"Wrong number of elements in list.");
+        Assert.assertEquals(twoUnmappedAtEnd,Arrays.asList(chr1,unmapped),"List sorted in wrong order");
+
+        List<GenomeLoc> twoUnmappedMixed = Arrays.asList(unmapped,chr1,unmapped);
+        twoUnmappedMixed = IntervalUtils.sortAndMergeIntervals(genomeLocParser,twoUnmappedMixed,IntervalMergingRule.OVERLAPPING_ONLY).toList();
+        Assert.assertEquals(twoUnmappedMixed.size(),2,"Wrong number of elements in list.");
+        Assert.assertEquals(twoUnmappedMixed,Arrays.asList(chr1,unmapped),"List sorted in wrong order");
+    }
 }