diff --git a/gprMax/geometry_primitives.pyx b/gprMax/geometry_primitives.pyx
index 18a7698b..821e0423 100644
--- a/gprMax/geometry_primitives.pyx
+++ b/gprMax/geometry_primitives.pyx
@@ -659,11 +659,12 @@ cpdef void build_sphere(int xc, int yc, int zc, float r, float dx, float dy, flo
                     build_voxel(i, j, k, numID, numIDx, numIDy, numIDz, averaging, solid, rigidE, rigidH, ID)
 
 
-cpdef void build_voxels_from_array(int xs, int ys, int zs, np.uint16_t[:, :, ::1] data, np.uint32_t[:, :, ::1] solid, np.int8_t[:, :, :, ::1] rigidE, np.int8_t[:, :, :, ::1] rigidH, np.uint32_t[:, :, :, ::1] ID):
+cpdef void build_voxels_from_array(int xs, int ys, int zs, int numexistmaterials, np.int16_t[:, :, ::1] data, np.uint32_t[:, :, ::1] solid, np.int8_t[:, :, :, ::1] rigidE, np.int8_t[:, :, :, ::1] rigidH, np.uint32_t[:, :, :, ::1] ID):
     """Builds Yee voxels by reading integers from an array.
         
     Args:
         xs, ys, zs (int): Cell coordinates of position of start of array in domain.
+        numexistmaterials (int): Number of existing materials in model prior to building voxels.
         data (memoryview): Access to array containing numeric IDs of voxels to create.
         solid, rigidE, rigidH, ID (memoryviews): Access to solid, rigid and ID arrays.
     """
@@ -697,5 +698,7 @@ cpdef void build_voxels_from_array(int xs, int ys, int zs, np.uint16_t[:, :, ::1
         for j in range(ys, yf):
             for k in range(zs, zf):
                 numID = data[i - xs, j - ys, k - zs]
-                build_voxel(i, j, k, numID, numID, numID, numID, False, solid, rigidE, rigidH, ID)
+                if numID >= 0:
+                    numID += numexistmaterials
+                    build_voxel(i, j, k, numID, numID, numID, numID, False, solid, rigidE, rigidH, ID)