last chance changes

portal_api.lua

@@ -2171,6 +2171,8 @@ end
 -- portal_name is optional, providing it allows existing portals on the surface to be reused.
 function nether.find_surface_target_y(target_x, target_z, portal_name)
 
+	assert(target_x ~= nil and target_z ~= nil, "Arguments `target_x` and `target_z` cannot be nil when calling find_surface_target_y()")
+
 	-- default to starting the search at -16 (probably underground) if we don't know the
 	-- surface, like paramat's original code from before get_spawn_level() was available:
 	-- https://github.com/minetest-mods/nether/issues/5#issuecomment-506983676
@@ -2226,6 +2228,7 @@ function nether.find_nearest_working_portal(portal_name, anchorPos, distance_lim
 
 	local portal_definition = nether.registered_portals[portal_name]
 	assert(portal_definition ~= nil, "find_nearest_working_portal() called with portal_name '" .. portal_name .. "', but no portal is registered with that name.")
+	assert(anchorPos         ~= nil, "Argument `anchorPos` cannot be nil when calling find_nearest_working_portal()")
 
 	local contenders = list_closest_portals(portal_definition, anchorPos, distance_limit, y_factor)