diff --git a/executor.go b/executor.go
index 7e046c11c..a2b6b8968 100644
--- a/executor.go
+++ b/executor.go
@@ -143,7 +143,6 @@ func (e *executor) Close() error {
 
 // Execute executes a PQL query.
 func (e *executor) Execute(ctx context.Context, index string, q *pql.Query, shards []uint64, opt *execOptions) (QueryResponse, error) {
-
 	span, ctx := tracing.StartSpanFromContext(ctx, "Executor.Execute")
 	span.LogKV("pql", q.String())
 	defer span.Finish()
@@ -1516,7 +1515,18 @@ func executeDistinctShardSet(ctx context.Context, qcx *Qcx, idx *Index, fieldNam
 	fragData, _, err := tx.ContainerIterator(index, fieldName, "standard", shard, 0)
 	switch errors.Cause(err) {
 	case ViewNotFound, FragmentNotFound:
-		return nil, nil
+		// It may seem reasonable to return `nil` here in the case where the
+		// fragment for this shard does not exist. The problem with doing that
+		// is that if this operation is being performed on a remote node, then
+		// this result is going to get serialized as a QueryResponse and sent
+		// back to the original, non-remote node. When this happens, the
+		// encodeRow/decodeRow logic replaces `nil` with an empty Row. An empty
+		// Row will cause problems during the union step of the reduce phase if
+		// it is the "left" side of the union, because then the resulting Row
+		// after the union will have blank Index and Field values. Here, we
+		// ensure that we send a non-nil Row with valid Index and Field values
+		// so that the union step doesn't cause problems.
+		return &Row{Index: index, Field: fieldName}, nil
 	case nil:
 	default:
 		return nil, errors.Wrap(err, "getting fragment data")
diff --git a/lattice b/lattice
index 36f453c1e..28c2313ec 160000
--- a/lattice
+++ b/lattice
@@ -1 +1 @@
-Subproject commit 36f453c1ea3bf86c546a8ad4a88f2a926724d683
+Subproject commit 28c2313ecfcd7e083d42d4e409483e968b4c421b
diff --git a/server/server.go b/server/server.go
index 291c986e1..530e4102a 100644
--- a/server/server.go
+++ b/server/server.go
@@ -24,6 +24,7 @@ import (
 	"context"
 	"crypto/tls"
 	"io"
+	"io/ioutil"
 	"log"
 	"math/rand"
 	"net"
@@ -31,6 +32,7 @@ import (
 	"os/signal"
 	"runtime"
 	"strconv"
+	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -152,6 +154,20 @@ func (m *Command) Start() (err error) {
 		return errors.Wrap(err, "setting up server")
 	}
 
+	if runtime.GOOS == "linux" {
+		result, err := ioutil.ReadFile("/proc/sys/vm/max_map_count")
+		if err != nil {
+			m.logger.Printf("Tried unsuccessfully to check system mmap limit: %v", err)
+		} else {
+			sysMmapLimit, err := strconv.ParseUint(strings.TrimSuffix(string(result), "\n"), 10, 64)
+			if err != nil {
+				m.logger.Printf("Tried unsuccessfully to check system mmap limit: %v", err)
+			} else if m.Config.MaxMapCount > sysMmapLimit {
+				m.logger.Printf("WARNING: Config max map limit (%v) is greater than current system limits (%v)", m.Config.MaxMapCount, sysMmapLimit)
+			}
+		}
+	}
+
 	// Set up networking (i.e. gossip)
 	err = m.setupNetworking()
 	if err != nil {