Negative instantanous frequency values in outputs of hht function

I am using hht function of MATLAB to evaluate instantanous frequency of a signal sig (a column vector of real numbers) using the followin:

[imf, residual] = emd(sig,'interpolation','pchip');
[hs, f, t, imfinsf, imfinse] = hht(imf,fs);

imfinsf has the same size as imf (as it was expected), but it contains some negaitve values. Negative values for frequency is physically meaningless.

How can I solve this problem?

Following figure shows an example: