z limits

Hello,

I am using R to run a program called GRASP - Generalized Regression 
Analysis and Spatial Prediction.

I am trying to draw a figure using GRASP, and I get the following 
error message in R:

Error in image.default(map, col = heat.colors(12)) :
	invalid z limits

What are z limits?  Can I manually set them?

Please let me know if you have any insight on this!

Thanks,
Jennifer Skene