diff --git a/algovivo/Vertices.js b/algovivo/Vertices.js
index 79046791..5b4d3801 100644
--- a/algovivo/Vertices.js
+++ b/algovivo/Vertices.js
@@ -22,6 +22,10 @@ class Vertices {
     throw new Error("use fixVertex instead");
   }
 
+  get fixedVertexId() {
+    return this._fixedVertexId;
+  }
+
   fixVertex(vertexId) {
     this._fixedVertexId = vertexId;
   }
diff --git a/algovivo/render/ViewportVertices.js b/algovivo/render/ViewportVertices.js
index 559f641d..1e1b972b 100644
--- a/algovivo/render/ViewportVertices.js
+++ b/algovivo/render/ViewportVertices.js
@@ -75,6 +75,7 @@ class ViewportVertices {
   }
 
   setVertexPos(i, p) {
+    if (i == null) throw new Error("vertex id required");
     const system = this.system;
     const pF32 = system.pos.slot.f32();
     const offset = i * 2;