VAST Error 501: NonLinear dimensions don't fit
The overlay creative is larger than the space the player allows — it won't scale or crop, so it isn't shown.
Overlays get a constrained region (commonly the lower third of the video). Error 501 means the creative's declared width/height exceed what the placement offers and the player won't shrink it.
Why error 501 happens
Creative sized for a bigger slot
A 728×90 unit sent to a player that caps overlays at 480×70, or a full-width banner on a small embedded player.
Fixed-size creative with scaling disabled
The unit declares scalable="false", so the player can't adapt it to the available region.
How to fix VAST error 501
- 1
Match declared dimensions to the placement spec
Read the placement's max overlay size and traffic a creative within it — or supply multiple sizes.
- 2
Allow scaling where design permits
Setting scalable="true" lets players fit the unit into smaller regions instead of rejecting it.
Seeing error 501 on a live tag?
Paste the tag URL or XML — the validator resolves the full wrapper chain, validates the schema, downloads every media file, and shows exactly which stage fails.
Validate your VAST tagVAST error 501 — frequently asked questions
What overlay size is safest?+
There is no universal size, but smaller-than-you-think wins: many players cap overlays around the lower 20% of the video area. Check each placement's spec rather than assuming IAB display sizes.