call a function from a package which is only imports not depends in R -

i'm make new package of r. in function of mine, called get_subnetwork package stringdb, while function get_subnetwork called function graph.data.frame internally , function graph.data.frame package igraph.

here want maek package stringdb , igraph imports not depends in description of new package. when ran code, showed

error in load() : not find function "graph.data.frame"