diff --git a/src/plot.js b/src/plot.js
index 2665465c66..20a72eaa13 100644
--- a/src/plot.js
+++ b/src/plot.js
@@ -16,6 +16,11 @@ export function plot(options = {}) {
 
   // Flatten any nested marks.
   const marks = options.marks === undefined ? [] : options.marks.flat(Infinity);
+  for (const mark of marks) {
+    if (mark.initialize === undefined) {
+      mark.initialize = () => ({ index: [], channels: [] });
+    }
+  }
 
   // A Map from Mark instance to an object of named channel values.
   const markChannels = new Map();