diff --git a/src/graph.ts b/src/graph.ts
index 9aafa61..78a9dfd 100644
--- a/src/graph.ts
+++ b/src/graph.ts
@@ -1008,22 +1008,25 @@ export class Graph<
   /**
    * Set node parent. If this operation causes a circle, it fails with an error.
    * @param id - ID of the child node.
-   * @param parent - ID of the parent node. If it is undefined, means unset parent for node with id.
+   * @param parent - ID of the parent node. If it is undefined or null, means unset parent for node with id.
    * @param treeKey - Which tree structure the relation is applied to.
    * @group TreeMethods
    */
-  public setParent(id: ID, parent?: ID, treeKey?: string) {
+  public setParent(id: ID, parent?: ID | null, treeKey?: string) {
     this.checkTreeExistence(treeKey);
 
-    const tree = this.treeIndices.get(treeKey)!;
+    const tree = this.treeIndices.get(treeKey);
+
+    if (!tree) return;
+
     const node = this.getNode(id);
     const oldParent = tree.parentMap.get(id);
 
     // Same parent id as old one, skip
     if (oldParent?.id === parent) return;
 
-    // New parent is undefined, unset parent for the node
-    if (parent === undefined) {
+    // New parent is undefined or null, unset parent for the node
+    if (parent === undefined || parent === null) {
       if (oldParent) {
         tree.childrenMap.get(oldParent.id)?.delete(node);
       }