index.js:33 Uncaught TypeError: Failed to execute 'createRadialGradient' on 'CanvasRenderingContext2D': The provided double value is non-finite.

var gradient=context.createRadialGradient(15 + i*30 + 2, 15 + j*30 - 2, 13, 15 + i*30 + 2, 15 + j*30 - 2, 0);        这行代码提示有错误，怎么回事？