diff --git a/light9/web/SyncedGraph.ts b/light9/web/SyncedGraph.ts
--- a/light9/web/SyncedGraph.ts
+++ b/light9/web/SyncedGraph.ts
@@ -197,17 +197,17 @@ export class SyncedGraph {
     this._autoDeps.graphChanged(patch);
   }
 
-  getObjectPatch(s: N3.NamedNode, p: N3.NamedNode, newObject: N3.Quad_Object, g: N3.NamedNode): Patch {
+  getObjectPatch(s: N3.NamedNode, p: N3.NamedNode, newObject: N3.Quad_Object | null, g: N3.NamedNode): Patch {
     // make a patch which removes existing values for (s,p,*,c) and
     // adds (s,p,newObject,c). Values in other graphs are not affected.
     const existing = this.graph.getQuads(s, p, null, g);
     return {
       dels: existing,
-      adds: [this.Quad(s, p, newObject, g)],
+      adds: newObject !== null ? [this.Quad(s, p, newObject, g)] : [],
     };
   }
 
-  patchObject(s: N3.NamedNode, p: N3.NamedNode, newObject: N3.Quad_Object, g: N3.NamedNode) {
+  patchObject(s: N3.NamedNode, p: N3.NamedNode, newObject: N3.Quad_Object | null, g: N3.NamedNode) {
     this.applyAndSendPatch(this.getObjectPatch(s, p, newObject, g));
   }