diff --git a/scalding-core/src/main/scala/com/twitter/scalding/typed/CoGrouped.scala b/scalding-core/src/main/scala/com/twitter/scalding/typed/CoGrouped.scala
index fed2756fb6..1b999e58f5 100644
--- a/scalding-core/src/main/scala/com/twitter/scalding/typed/CoGrouped.scala
+++ b/scalding-core/src/main/scala/com/twitter/scalding/typed/CoGrouped.scala
@@ -163,7 +163,14 @@ trait CoGrouped[K, +R] extends KeyedListLike[K, R, CoGrouped] with CoGroupable[K
       def reducers = self.reducers
       def keyOrdering = self.keyOrdering
       def joinFunction = { (k: K, leftMost: Iterator[CTuple], joins: Seq[Iterable[CTuple]]) =>
-        fn(k, joinF(k, leftMost, joins))
+        val joined = joinF(k, leftMost, joins)
+        /*
+         * After the join, if the key has no values, don't present it to the mapGroup
+         * function. Doing so would break the invariant:
+         *
+         * a.join(b).toTypedPipe.group.mapGroup(fn) == a.join(b).mapGroup(fn)
+         */
+        if (joined.nonEmpty) fn(k, joined) else Iterator.empty
       }
     }
   }
diff --git a/scalding-core/src/main/scala/com/twitter/scalding/typed/Grouped.scala b/scalding-core/src/main/scala/com/twitter/scalding/typed/Grouped.scala
index 54434982c2..3b422afb59 100644
--- a/scalding-core/src/main/scala/com/twitter/scalding/typed/Grouped.scala
+++ b/scalding-core/src/main/scala/com/twitter/scalding/typed/Grouped.scala
@@ -260,7 +260,11 @@ case class ValueSortedReduce[K, V1, V2](
   override def mapGroup[V3](fn: (K, Iterator[V2]) => Iterator[V3]) = {
     // don't make a closure
     val localRed = reduceFn
-    val newReduce = { (k: K, iter: Iterator[V1]) => fn(k, localRed(k, iter)) }
+    val newReduce = { (k: K, iter: Iterator[V1]) =>
+      val step1 = localRed(k, iter)
+      // Only pass non-Empty iterators to subsequent functions
+      if (step1.nonEmpty) fn(k, step1) else Iterator.empty
+    }
     ValueSortedReduce[K, V1, V3](
       keyOrdering, mapped, valueSort, newReduce, reducers)
   }
@@ -293,7 +297,11 @@ case class IteratorMappedReduce[K, V1, V2](
   override def mapGroup[V3](fn: (K, Iterator[V2]) => Iterator[V3]) = {
     // don't make a closure
     val localRed = reduceFn
-    val newReduce = { (k: K, iter: Iterator[V1]) => fn(k, localRed(k, iter)) }
+    val newReduce = { (k: K, iter: Iterator[V1]) =>
+      val step1 = localRed(k, iter)
+      // Only pass non-Empty iterators to subsequent functions
+      if (step1.nonEmpty) fn(k, step1) else Iterator.empty
+    }
     copy(reduceFn = newReduce)
   }
 
diff --git a/scalding-core/src/main/scala/com/twitter/scalding/typed/KeyedList.scala b/scalding-core/src/main/scala/com/twitter/scalding/typed/KeyedList.scala
index 7b7f3f8e9c..2ac3e64aae 100644
--- a/scalding-core/src/main/scala/com/twitter/scalding/typed/KeyedList.scala
+++ b/scalding-core/src/main/scala/com/twitter/scalding/typed/KeyedList.scala
@@ -70,6 +70,10 @@ trait KeyedListLike[K, +T, +This[K, +T] <: KeyedListLike[K, T, This]]
    * Prefer this to toList, when you can avoid accumulating the whole list in memory.
    * Prefer sum, which is partially executed map-side by default.
    * Use mapValueStream when you don't care about the key for the group.
+   *
+   * Iterator is always Non-empty.
+   * Note, any key that has all values removed will not appear in subsequent
+   * .mapGroup/mapValueStream
    */
   def mapGroup[V](smfn: (K, Iterator[T]) => Iterator[V]): This[K, V]
 
diff --git a/scalding-core/src/test/scala/com/twitter/scalding/TypedPipeTest.scala b/scalding-core/src/test/scala/com/twitter/scalding/TypedPipeTest.scala
index e8e8a6181c..f9c5afedbf 100644
--- a/scalding-core/src/test/scala/com/twitter/scalding/TypedPipeTest.scala
+++ b/scalding-core/src/test/scala/com/twitter/scalding/TypedPipeTest.scala
@@ -1103,6 +1103,29 @@ class TypedSelfLeftCrossTest extends Specification {
   }
 }
 
+class JoinMapGroupJob(args: Args) extends Job(args) {
+  def r1 = TypedPipe.from(Seq((1, 10)))
+  def r2 = TypedPipe.from(Seq((1, 1), (2, 2), (3, 3)))
+  r1.groupBy(_._1).join(r2.groupBy(_._1))
+    .mapGroup { case (a, b) => Iterator("a") }
+    .write(TypedTsv("output"))
+}
+class JoinMapGroupJobTest extends Specification {
+  import Dsl._
+  noDetailedDiffs()
+
+  "A JoinMapGroupJob" should {
+    JobTest(new JoinMapGroupJob(_))
+      .sink[(Int, String)](TypedTsv[(Int, String)]("output")) { outBuf =>
+        "not duplicate keys" in {
+          outBuf.toList must be_==(List((1, "a")))
+        }
+      }
+      .run
+      .finish
+  }
+}
+
 class TypedSketchJoinJob(args: Args) extends Job(args) {
   val zero = TypedPipe.from(TypedTsv[(Int, Int)]("input0"))
   val one = TypedPipe.from(TypedTsv[(Int, Int)]("input1"))