leaflet heatmap - crash when zoom out

[luojinzhang]

leaflet heatmap crashes when zoom out to level 3 of leaflet map. The error is from this line.
shadowCtx.drawImage(tpl, rectX, rectY);
Failed to execute 'drawImage' on 'CanvasRenderingContext2D': The image argument is a canvas element with a width or height of 0.
Current workaround is to set min zoom = 4 only.