diff --git a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
index 40cf7196c..a46c4ea3e 100644
--- a/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
+++ b/protected/gatk-tools-protected/src/main/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngine.java
@@ -348,7 +348,8 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
      * @param originalCalls    the original unphased calls
      * @param haplotypeMap     mapping from alternate allele to the set of haplotypes that contain that allele
      * @param totalAvailableHaplotypes the total number of possible haplotypes used in calling
-     * @param phaseSetMapping  the map to populate in this method
+     * @param phaseSetMapping  the map to populate in this method;
+     *                         note that it is okay for this method NOT to populate the phaseSetMapping at all (e.g. in an impossible-to-phase situation)
      * @return the next incremental unique index
      */
     protected static int constructPhaseSetMapping(final List<VariantContext> originalCalls,
@@ -366,6 +367,8 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
             if ( haplotypesWithCall.isEmpty() )
                 continue;
 
+            final boolean callIsOnAllHaps = haplotypesWithCall.size() == totalAvailableHaplotypes;
+
             for ( int j = i+1; j < numCalls; j++ ) {
                 final VariantContext comp = originalCalls.get(j);
                 final Set<Haplotype> haplotypesWithComp = haplotypeMap.get(comp);
@@ -374,12 +377,19 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
 
                 // if the variants are together on all haplotypes, record that fact.
                 // another possibility is that one of the variants is on all possible haplotypes (i.e. it is homozygous).
-                final boolean callIsOnAllHaps = haplotypesWithCall.size() == totalAvailableHaplotypes;
                 final boolean compIsOnAllHaps = haplotypesWithComp.size() == totalAvailableHaplotypes;
                 if ( (haplotypesWithCall.size() == haplotypesWithComp.size() && haplotypesWithCall.containsAll(haplotypesWithComp)) || callIsOnAllHaps || compIsOnAllHaps ) {
 
                     // create a new group if these are the first entries
                     if ( ! phaseSetMapping.containsKey(call) ) {
+                        // note that if the comp is already in the map then that is very bad because it means that there is
+                        // another variant that is in phase with the comp but not with the call.  Since that's an un-phasable
+                        // situation, we should abort if we encounter it.
+                        if ( phaseSetMapping.containsKey(comp) ) {
+                            phaseSetMapping.clear();
+                            return 0;
+                        }
+
                         phaseSetMapping.put(call, new Pair<>(uniqueCounter, callIsOnAllHaps ? "1|1" : "0|1"));
                         phaseSetMapping.put(comp, new Pair<>(uniqueCounter, compIsOnAllHaps ? "1|1" : "0|1"));
                         uniqueCounter++;
@@ -399,6 +409,14 @@ public class HaplotypeCallerGenotypingEngine extends GenotypingEngine<HaplotypeC
                     if ( intersection.isEmpty() ) {
                         // create a new group if these are the first entries
                         if ( ! phaseSetMapping.containsKey(call) ) {
+                            // note that if the comp is already in the map then that is very bad because it means that there is
+                            // another variant that is in phase with the comp but not with the call.  Since that's an un-phasable
+                            // situation, we should abort if we encounter it.
+                            if ( phaseSetMapping.containsKey(comp) ) {
+                                phaseSetMapping.clear();
+                                return 0;
+                            }
+
                             phaseSetMapping.put(call, new Pair<>(uniqueCounter, "0|1"));
                             phaseSetMapping.put(comp, new Pair<>(uniqueCounter, "1|0"));
                             uniqueCounter++;
diff --git a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
index 635a5997d..11a902b08 100644
--- a/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
+++ b/protected/gatk-tools-protected/src/test/java/org/broadinstitute/gatk/tools/walkers/haplotypecaller/HaplotypeCallerGenotypingEngineUnitTest.java
@@ -439,6 +439,10 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         pos24.setEventMap(new EventMap(Arrays.asList(vc2, vc4)));
         pos24.getEventMap().put(2, vc2);
         pos24.getEventMap().put(4, vc4);
+        final Haplotype pos34 = new Haplotype("AACCA".getBytes());
+        pos34.setEventMap(new EventMap(Arrays.asList(vc3, vc4)));
+        pos34.getEventMap().put(3, vc3);
+        pos34.getEventMap().put(4, vc4);
         final Haplotype pos234 = new Haplotype("ACCCA".getBytes());
         pos234.setEventMap(new EventMap(Arrays.asList(vc2, vc3, vc4)));
         pos234.getEventMap().put(2, vc2);
@@ -509,6 +513,20 @@ public class HaplotypeCallerGenotypingEngineUnitTest extends BaseTest {
         haplotypeMap.put(vc4, haplotypes4het);
         tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 2, 3, 1, 2, 0});
 
+        // test no phased variants around a hom
+        final Set<Haplotype> haplotypes2incomplete = new HashSet<>();
+        haplotypes2incomplete.add(pos24);
+        final Set<Haplotype> haplotypes3incomplete = new HashSet<>();
+        haplotypes3incomplete.add(pos34);
+        final Set<Haplotype> haplotypes4complete = new HashSet<>();
+        haplotypes4complete.add(pos24);
+        haplotypes4complete.add(pos34);
+        haplotypes4complete.add(pos234);
+        haplotypeMap.put(vc2, haplotypes2incomplete);
+        haplotypeMap.put(vc3, haplotypes3incomplete);
+        haplotypeMap.put(vc4, haplotypes4complete);
+        tests.add(new Object[]{calls, new HashMap<>(haplotypeMap), 0, 0, 0, 0, 0});
+
         return tests.toArray(new Object[][]{});
     }