diff --git a/packages/mermaid/src/diagrams/user-journey/journeyRenderer.ts b/packages/mermaid/src/diagrams/user-journey/journeyRenderer.ts
index bb3386bfc..75a441d31 100644
--- a/packages/mermaid/src/diagrams/user-journey/journeyRenderer.ts
+++ b/packages/mermaid/src/diagrams/user-journey/journeyRenderer.ts
@@ -22,6 +22,10 @@ function drawActorLegend(diagram) {
   maxWidth = 0;
   let yPos = 60;
 
+  if (isNaN(maxLabelWidth)) {
+    throw new Error('maxLabelWidth must be a number');
+  }
+
   Object.keys(actors).forEach((person) => {
     const colour = actors[person].color;
     const circleData = {