diff --git a/gprMax/input_cmds_geometry.py b/gprMax/input_cmds_geometry.py
index 872f23e1..16dba2f3 100644
--- a/gprMax/input_cmds_geometry.py
+++ b/gprMax/input_cmds_geometry.py
@@ -109,7 +109,7 @@ def process_geometrycmds(geometry, G):
                 G.solid[xs:xs + data.shape[0], ys:ys + data.shape[1], zs:zs + data.shape[2]] = data + numexistmaterials
                 G.rigidE[:, xs:xs + rigidE.shape[1], ys:ys + rigidE.shape[2], zs:zs + rigidE.shape[3]] = rigidE
                 G.rigidH[:, xs:xs + rigidH.shape[1], ys:ys + rigidH.shape[2], zs:zs + rigidH.shape[3]] = rigidH
-                G.ID[:, xs:xs + ID.shape[1], ys:ys + ID.shape[2], zs:zs + ID.shape[3]] = ID
+                G.ID[:, xs:xs + ID.shape[1], ys:ys + ID.shape[2], zs:zs + ID.shape[3]] = ID + numexistmaterials
                 if G.messages:
                     tqdm.write('Geometry objects from file {} inserted at {:g}m, {:g}m, {:g}m, with corresponding materials file {}.'.format(geofile, xs * G.dx, ys * G.dy, zs * G.dz, matfile))
             except KeyError: