Cannot merge two PointM shapefiles

[TestMethod]
public void MergeM()
{
    const string sf1Location = @"shp1_point_m\SHP1_POINT_M.shp";
    const string sf2Location = @"shp2_point_m\SHP2_POINT_M.shp";

    var sf1 = new Shapefile();
    if (!sf1.Open(sf1Location)) Assert.Fail("Can't open " + sf1Location + " Error: " + sf1.ErrorMsg[sf1.LastErrorCode]);

    var sf2 = new Shapefile();
    if (!sf2.Open(sf2Location)) Assert.Fail("Can't open " + sf2Location + " Error: " + sf2.ErrorMsg[sf2.LastErrorCode]);

    var sfMerged = sf1.Merge(false, sf2, false);
    Assert.IsNotNull(sfMerged, "Merge failed");
    Assert.AreEqual(2, sfMerged.NumShapes, "Incorrect number of shapes");
}