Dlmopen returns NULL where as dlopen works

I have a third party shared library. When I perform below operation with dlmopen it returns NULL:

void* module1 = dlmopen(LM_ID_NEWLM, s->soPath,  RTLD_LAZY);
if(!module1) {
    printf("dlmopen doesnt work for %s\n", s->soPath );
    assert(0);
}

whereas if I call dlopen like below it works

void* module1 = dlopen(s->soPath , RTLD_LAZY);
assert(module1);

Can anyone help what can be wrong with the third party shared library?