command for specifying markerfacecolour and marker size

Hi i am trying to use the command below to create a graph with a red solid line, that has circular markers which have a face colour green and a size of 4.

plot(t,c,'-or','markerfacecolor','g','markersize','4)

where t is (0:1:60) and c=c0*exp((-r/v)*t), r,v and c0 are all defined

However i am receiving the error. "error using plot, value must be numeric"

Any help with understanding the reason for this error would be appreciated.

thanks