Example warning text
Zero datapoints supplied to function Regression, i.e., the «I» index is length zero.
When calling Regression(y, B, I, k), this warning will occur if index «I» has zero data points -- in which case there are zero data points to fit a curve to.
This can also occur when Null values exist in «y», since the data points where «y» is Null are ignored. If all data points are ignored as a result, then again, you may end up with zero data points even though «I» is not of zero length.
When you ignore this warning, the result will have all coefficients equal to zero. (The constant y = 0 is as good a fit as any curve to the zero data points).