diff --git a/graph/iterator/materialize_iterator.go b/graph/iterator/materialize_iterator.go
index 2e41969..1b448b3 100644
--- a/graph/iterator/materialize_iterator.go
+++ b/graph/iterator/materialize_iterator.go
@@ -95,6 +95,12 @@ func (it *Materialize) TagResults(dst map[string]graph.Value) {
 func (it *Materialize) Clone() graph.Iterator {
 	out := NewMaterialize(it.subIt.Clone())
 	out.tags.CopyFrom(it)
+	if it.hasRun {
+		out.hasRun = true
+		out.aborted = it.aborted
+		out.values = it.values
+		out.containsMap = it.containsMap
+	}
 	return out
 }
 
diff --git a/graph/triplestore.go b/graph/triplestore.go
index c099d47..bfdf62f 100644
--- a/graph/triplestore.go
+++ b/graph/triplestore.go
@@ -28,14 +28,17 @@ import (
 	"github.com/google/cayley/quad"
 )
 
-// Defines an opaque "triple store value" type. However the backend wishes to
-// implement it, a Value is merely a token to a triple or a node that the backing
-// store itself understands, and the base iterators pass around.
+// Value defines an opaque "triple store value" type. However the backend wishes
+// to implement it, a Value is merely a token to a triple or a node that the
+// backing store itself understands, and the base iterators pass around.
 //
 // For example, in a very traditional, graphd-style graph, these are int64s
 // (guids of the primitives). In a very direct sort of graph, these could be
 // pointers to structs, or merely triples, or whatever works best for the
 // backing store.
+//
+// These must be comparable, ie, not arrays or maps, as they may be used as keys
+// for maps.
 type Value interface{}
 
 type TripleStore interface {