4 files changed, 58 insertions, 2 deletions
diff --git a/jstests/aggregation/bugs/server14691.js b/jstests/aggregation/bugs/server14691.js
new file mode 100644
index 00000000000..0ba010ac41a
--- /dev/null
+++ b/jstests/aggregation/bugs/server14691.js
@@ -0,0 +1,52 @@
+// SERVER-14691: $avg aggregator should return null when it receives no input.
+(function() {
+    'use strict';
+
+    var coll = db.accumulate_avg_sum_null;
+
+    // Test the $avg aggregator.
+    coll.drop();
+
+    // Null cases.
+    assert.writeOK(coll.insert({a: 1, b: 2, c: 'string', d: null}));
+
+    // Missing field.
+    var pipeline = [{$group: {_id: '$a', avg: {$avg: '$missing'}}}];
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: null}]);
+
+    // Non-numeric field.
+    pipeline = [{$group: {_id: '$a', avg: {$avg: '$c'}}}];
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: null}]);
+
+    // Field with value of null.
+    pipeline = [{$group: {_id: '$a', avg: {$avg: '$d'}}}];
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: null}]);
+
+    // All three.
+    coll.insert({a: 1, d: 'string'});
+    coll.insert({a: 1});
+    pipeline = [{$group: {_id: '$a', avg: {$avg: '$d'}}}];
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: null}]);
+
+    // Non-null cases.
+    coll.drop();
+    assert.writeOK(coll.insert({a: 1, b: 2}));
+    pipeline = [{$group: {_id: '$a', avg: {$avg: '$b'}}}];
+
+    // One field.
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: 2}]);
+
+    // Two fields.
+    assert.writeOK(coll.insert({a: 1, b: 4}));
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: 3}]);
+
+    // Average of zero should still work.
+    assert.writeOK(coll.insert({a: 1, b: -6}));
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: 0}]);
+
+    // Missing, null, or non-numeric fields should not error or affect the average.
+    assert.writeOK(coll.insert({a: 1}));
+    assert.writeOK(coll.insert({a: 1, b: 'string'}));
+    assert.writeOK(coll.insert({a: 1, b: null}));
+    assert.eq(coll.aggregate(pipeline).toArray(), [{_id: 1, avg: 0}]);
+}());
diff --git a/jstests/aggregation/testshard1.js b/jstests/aggregation/testshard1.js
index 81f27e51830..71271ca0af5 100644
--- a/jstests/aggregation/testshard1.js
+++ b/jstests/aggregation/testshard1.js
@@ -129,6 +129,10 @@ jsTestLog('sum of an arithmetic progression S(n) = (n/2)(a(1) + a(n));');
 assert.eq(a2[0].total, (nItems/2)*(1 + nItems),
        'agg sharded test counter sum failed');
 
+jsTestLog('A group combining all documents into one, averaging a null field.');
+assert.eq(aggregateOrdered(db.ts1, [{$group: {_id: null, avg: {$avg: "$missing"}}}]),
+          [{_id: null, avg: null}]);
+
 jsTestLog('an initial group starts the group in the shards, and combines them in mongos');
 var a3 = aggregateOrdered(db.ts1, [
     { $group: {
diff --git a/src/mongo/db/pipeline/accumulator_avg.cpp b/src/mongo/db/pipeline/accumulator_avg.cpp
index cf2d5c9b677..06dd585bdf7 100644
--- a/src/mongo/db/pipeline/accumulator_avg.cpp
+++ b/src/mongo/db/pipeline/accumulator_avg.cpp
@@ -72,7 +72,7 @@ intrusive_ptr<Accumulator> AccumulatorAvg::create() {
 Value AccumulatorAvg::getValue(bool toBeMerged) const {
     if (!toBeMerged) {
         if (_count == 0)
-            return Value(0.0);
+            return Value(BSONNULL);
 
         return Value(_total / static_cast<double>(_count));
     } else {
diff --git a/src/mongo/db/pipeline/accumulator_test.cpp b/src/mongo/db/pipeline/accumulator_test.cpp
index e3e6bd04d87..9bef088316d 100644
--- a/src/mongo/db/pipeline/accumulator_test.cpp
+++ b/src/mongo/db/pipeline/accumulator_test.cpp
@@ -84,7 +84,7 @@ class None : public Base {
 public:
     void run() {
         createAccumulator();
-        ASSERT_EQUALS(0, accumulator()->getValue(false).getDouble());
+        ASSERT_EQUALS(Value(BSONNULL), accumulator()->getValue(false));
     }
 };