fixup! Add support for top level aggregates

query/graphql/planner/top.go

@@ -11,6 +11,8 @@
 package planner
 
 import (
+	"errors"
+
 	"github.com/sourcenetwork/defradb/core"
 	"github.com/sourcenetwork/defradb/query/graphql/mapper"
 	parserTypes "github.com/sourcenetwork/defradb/query/graphql/parser/types"
@@ -146,10 +148,13 @@ func (n *topLevelNode) Next() (bool, error) {
 			// This Next will always return a value, as it's source is this node!
 			// Even if it adds nothing to the current currentValue, it should still
 			// yield it unchanged.
-			_, err := child.Next()
+			hasChild, err := child.Next()
 			if err != nil {
 				return false, err
 			}
+			if !hasChild {
+				return false, errors.New("Expected child value, however none was yielded")
+			}
 
 			n.currentValue = child.Value()
 		}