diff --git a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManager.java b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManager.java
index 4ac4f5739..cd4f359b4 100644
--- a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManager.java
+++ b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManager.java
@@ -279,7 +279,7 @@ public class ConstrainedMateFixingManager {
         // fix mates, as needed
         // Since setMateInfo can move reads, we potentially need to remove the mate, and requeue
         // it to ensure proper sorting
-        if ( newRead.getReadPairedFlag() && !newRead.getNotPrimaryAlignmentFlag() ) {
+        if ( isMateFixableRead(newRead) ) {
             SAMRecordHashObject mate = forMateMatching.get(newRead.getReadName());
             if ( mate != null ) {
                 // 1. Frustratingly, Picard's setMateInfo() method unaligns (by setting the reference contig
@@ -307,13 +307,9 @@ public class ConstrainedMateFixingManager {
                             reQueueMate = false;
                     }
 
-                    // we've already seen our mate -- set the mate info and remove it from the map
-                    // Via Nils Homer:
-                    //   There will be two SamPairUtil.setMateInfo functions.  The default will not update the mate
-                    //   cigar tag; in fact, it will remove it if it is present.  An alternative SamPairUtil.setMateInfo
-                    //   function takes a boolean as an argument ("addMateCigar") and will add/update the mate cigar if
-                    //   set to true.  This is the one you want to use.
-                    SamPairUtil.setMateInfo(mate.record, newRead, null, true);
+                    // we've already seen our mate -- set the mate info and remove it from the map;
+                    // add/update the mate cigar if appropriate
+                    SamPairUtil.setMateInfo(mate.record, newRead, true);
                     if ( reQueueMate ) waitingReads.add(mate.record);
                 }
 
@@ -364,6 +360,16 @@ public class ConstrainedMateFixingManager {
         }
     }
 
+    /**
+     * Is the given read one for which we can fix its mate?
+     *
+     * @param read  the read
+     * @return true if we could fix its mate, false otherwise
+     */
+    protected boolean isMateFixableRead(final SAMRecord read) {
+        return read.getReadPairedFlag() && !read.isSecondaryOrSupplementary();
+    }
+
     /**
      * @param read  the read
      * @return true if the read shouldn't be moved given the constraints of this SAMFileWriter
diff --git a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManagerUnitTest.java b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManagerUnitTest.java
index 4f5453fdb..d35536df3 100644
--- a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManagerUnitTest.java
+++ b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/indels/ConstrainedMateFixingManagerUnitTest.java
@@ -52,7 +52,9 @@
 package org.broadinstitute.gatk.tools.walkers.indels;
 
 import htsjdk.samtools.SAMFileHeader;
+import htsjdk.samtools.SAMFileWriter;
 import htsjdk.samtools.SAMRecord;
+import htsjdk.samtools.util.ProgressLoggerInterface;
 import org.broadinstitute.gatk.utils.BaseTest;
 import org.broadinstitute.gatk.utils.GenomeLocParser;
 import org.broadinstitute.gatk.utils.sam.ArtificialSAMUtils;
@@ -61,6 +63,7 @@ import org.testng.Assert;
 import org.testng.annotations.BeforeClass;
 import org.testng.annotations.Test;
 
+import java.util.ArrayList;
 import java.util.List;
 
 
@@ -136,4 +139,60 @@ public class ConstrainedMateFixingManagerUnitTest extends BaseTest {
 
         Assert.assertTrue(manager.forMateMatching.containsKey("foo"));
     }
+
+    @Test
+    public void testSupplementaryAlignmentsDoNotCauseBadMateFixing() {
+        final List<GATKSAMRecord> properReads = ArtificialSAMUtils.createPair(header, "foo", 1, 1000, 2000, true, false);
+        final GATKSAMRecord read1 = properReads.get(0);
+        read1.setFlags(99);   // first in pair, negative strand
+
+        final GATKSAMRecord read2 = properReads.get(1);
+        read2.setFlags(161);   // second in pair, mate negative strand
+
+        final GATKSAMRecord read2Supp = new GATKSAMRecord(read2);
+        read2Supp.setReadName("foo");
+        read2Supp.setFlags(2209);   // second in pair, mate negative strand, supplementary
+        read2Supp.setAlignmentStart(100);
+        read2Supp.setMateAlignmentStart(1000);
+
+        final DummyWriter writer = new DummyWriter();
+        final ConstrainedMateFixingManager manager = new ConstrainedMateFixingManager(writer, genomeLocParser, 10000, 200, 10000);
+        manager.addRead(read2Supp, false, false);
+        manager.addRead(read1, false, false);
+        manager.addRead(read2, false, false);
+        manager.close(); // "write" the reads to our dummy writer
+
+        // check to make sure that none of the mate locations were changed, which is the problem brought to us by a user
+        for ( final SAMRecord read : writer.reads ) {
+            final int start = read.getAlignmentStart();
+            switch (start) {
+                case 100:
+                    Assert.assertEquals(read.getMateAlignmentStart(), 1000);
+                    break;
+                case 1000:
+                    Assert.assertEquals(read.getMateAlignmentStart(), 2000);
+                    break;
+                case 2000:
+                    Assert.assertEquals(read.getMateAlignmentStart(), 1000);
+                    break;
+                default:
+                    Assert.assertTrue(false, "We saw a read located at the wrong position");
+            }
+        }
+    }
+
+    private class DummyWriter implements SAMFileWriter {
+
+        public List<SAMRecord> reads;
+
+        public DummyWriter() { reads = new ArrayList<>(10); }
+
+        public void addAlignment(final SAMRecord alignment) { reads.add(alignment);}
+
+        public SAMFileHeader getFileHeader() { return null; }
+
+        public void setProgressLogger(final ProgressLoggerInterface progress) {}
+
+        public void close() {}
+    }
 }
\ No newline at end of file