increase the separation of two lines in double bonds using chemfig package

I was trying to draw a molecule using chemfig package where i would like to increase the spacing between the two lines of double bond. From the manual of chemfig, i came to know that by using the option double bond sep = 3pt, we can do it. But when i use the same in my file, i get the error Package pgfkeys Error: I do not know the key ‘/tikz/double bond sep’, to which you passed ‘3pt’, and I am going to ignore it. Perhaps you misspelled it.
I don’t know how to resolve this.
MY MWE is as follow:

    documentclass[12pt,a4paper]{article}
usepackage{chemfig}
begin{document}
chemfig{B=C}~~~~~~
        chemfig[double bond sep = 3pt]{B=C}
end{document}

I had also tried another MWE as follows which also gives same error:

documentclass[12pt,a4paper]{article}
    usepackage{chemfig}
    begin{document}
    chemfig{B=C}~~~~~~
            chemfig{B=[,,,, double bond sep = 3pt]C}
    end{document}