diff --git a/src/main/scala/scala/collection/decorators/MapDecorator.scala b/src/main/scala/scala/collection/decorators/MapDecorator.scala
index bb7a90e..e3c7aa3 100644
--- a/src/main/scala/scala/collection/decorators/MapDecorator.scala
+++ b/src/main/scala/scala/collection/decorators/MapDecorator.scala
@@ -59,16 +59,16 @@ class MapDecorator[C, M <: IsMap[C]](coll: C)(implicit val map: M) {
     */
   def mergeByKeyWith[W, X, That](other: Map[map.K, W])(f: PartialFunction[(Option[map.V], Option[W]), X])(implicit bf: BuildFrom[C, (map.K, X), That]): That = {
     val b = bf.newBuilder(coll)
-    val traversed = mutable.Set.empty[W]
+    val traversed = mutable.Set.empty[map.K]
     val pf = f.lift
     for {
       (k, v) <- map(coll)
-      x <- pf(other.get(k).fold[(Option[map.V], Option[W])]((Some(v), None)){ w => traversed += w; (Some(v), Some(w)) })
+      x <- pf(other.get(k).fold[(Option[map.V], Option[W])]((Some(v), None)){ w => traversed += k; (Some(v), Some(w)) })
     } {
       b += k -> x
     }
     for {
-      (k, w) <- other if !traversed(w)
+      (k, w) <- other if !traversed(k)
       x <- pf((None, Some(w)))
     } {
       b += k -> x
diff --git a/src/test/scala/scala/collection/decorators/MapDecoratorTest.scala b/src/test/scala/scala/collection/decorators/MapDecoratorTest.scala
index 8d8e474..3aae2fa 100644
--- a/src/test/scala/scala/collection/decorators/MapDecoratorTest.scala
+++ b/src/test/scala/scala/collection/decorators/MapDecoratorTest.scala
@@ -74,4 +74,92 @@ class MapDecoratorTest {
 //    Assert.assertEquals(expected, zipped2)
   }
 
+  @Test
+  def mergingByKeyPerformsFullOuterJoin(): Unit = {
+    val arthur = "arthur.txt"
+
+    val tyson = "tyson.txt"
+
+    val sandra = "sandra.txt"
+
+    val allKeys = Set(arthur, tyson, sandra)
+
+    val sharedValue = 1
+
+    val ourChanges = Map(
+      (
+        arthur,
+        sharedValue
+      ),
+      (
+        tyson,
+        2
+      )
+    )
+
+    locally {
+      // In this test case, none of the associated values collide across keys...
+
+      val theirChanges = Map(
+        (
+          arthur,
+          sharedValue
+        ),
+        (
+          sandra,
+          3
+        )
+      )
+
+      Assert.assertEquals("Expect the same keys to appear in the join taken either way around.", ourChanges.mergeByKey(theirChanges).keySet, theirChanges
+        .mergeByKey(ourChanges)
+        .keySet)
+
+      Assert.assertTrue("Expect the same associated values to appear in the join taken either way around, albeit swapped around and not necessarily in the same key order.",
+        ourChanges
+          .mergeByKey(theirChanges)
+          .values
+          .map(_.swap)
+          .toList
+          .sorted
+          .sameElements(theirChanges.mergeByKey(ourChanges).values.toList.sorted))
+
+      Assert.assertEquals("Expect all the keys to appear in an outer join.", ourChanges.mergeByKey(theirChanges).keySet, allKeys)
+
+      Assert.assertEquals("Expect all the keys to appear in an outer join.", theirChanges.mergeByKey(ourChanges).keySet, allKeys)
+    }
+
+    locally {
+      // In this test case, associated values collide across keys...
+
+      val theirChangesRedux = Map(
+        (
+          arthur,
+          sharedValue
+        ),
+        (
+          sandra,
+          sharedValue
+        )
+      )
+
+      Assert.assertEquals("Expect the same keys to appear in the join taken either way around.", ourChanges.mergeByKey(theirChangesRedux).keySet, theirChangesRedux
+        .mergeByKey(ourChanges)
+        .keySet)
+
+      Assert.assertTrue("Expect the same associated values to appear in the join taken either way around, albeit swapped around and not necessarily in the same key order.",
+        ourChanges
+          .mergeByKey(theirChangesRedux)
+          .values
+          .map(_.swap)
+          .toList
+          .sorted
+          .sameElements(theirChangesRedux.mergeByKey(ourChanges).values.toList.sorted))
+
+      Assert.assertEquals("Expect all the keys to appear in an outer join.", ourChanges.mergeByKey(theirChangesRedux).keySet, allKeys)
+
+      Assert.assertEquals("Expect all the keys to appear in an outer join.", theirChangesRedux.mergeByKey(ourChanges).keySet, allKeys)
+    }
+  }
+
 }