diff --git a/Project.toml b/Project.toml
index f0093ce..8592228 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "LayeredLayouts"
 uuid = "f4a74d36-062a-4d48-97cd-1356bad1de4e"
 authors = ["Frames White <oxinabox@ucc.asn.au> and contributors"]
-version = "0.2.9"
+version = "0.2.10"
 
 [deps]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
diff --git a/src/graph_properties.jl b/src/graph_properties.jl
index fe455dd..bc1a96c 100644
--- a/src/graph_properties.jl
+++ b/src/graph_properties.jl
@@ -7,7 +7,7 @@ sinks(graph) = findall(iszero, outdegree(graph))
 
 function longest_paths(graph, roots)
     dag_or_error(graph)
-    dists = zeros(nv(graph))
+    dists = zeros(Int, nv(graph))
     pending = [0 => r for r in roots]
     while(!isempty(pending))
         depth, node = pop!(pending)
diff --git a/src/layering.jl b/src/layering.jl
index b514232..4f7859d 100644
--- a/src/layering.jl
+++ b/src/layering.jl
@@ -3,7 +3,11 @@
 function layer_by_longest_path_to_source(graph, force_layer)
     dists = longest_paths(graph, sources(graph))
     force_layers!(graph, dists, force_layer)
-    layer_groups = collect.(IterTools.groupby(i->dists[i], sort(vertices(graph), by=i->dists[i])))
+    layer_groups = [Int[] for d in 1:maximum(dists)]
+    for (ii, d) in enumerate(dists)
+        push!(layer_groups[d], ii)
+    end
+
     return layer_groups
 end
 
diff --git a/src/zarate.jl b/src/zarate.jl
index 79a26fb..b48f9ab 100644
--- a/src/zarate.jl
+++ b/src/zarate.jl
@@ -42,8 +42,10 @@ different nodes as x and y coordinates.
 Optional arguments:
 
 `force_layer`: Vector{Pair{Int, Int}}
-    specifies the layer for each node
-    e.g. [3=>1, 5=>5] specifies layer 1 for node 3 and layer 5 to node 5
+    specifies the layer for each node e.g. [3=>1, 5=>5] specifies layer 1 for node 3 and
+    layer 5 to node 5. If not specified, layer indices are calculated by longest path to
+    source vertices, with child vertex' layers following parents', and with layer indices
+    assigned consecutively from 1.
 
 `force_order`: Vector{Pair{Int, Int}}
     this vector forces the ordering of the nodes in each layer,
@@ -278,6 +280,7 @@ function assign_coordinates(layout, graph, layer2nodes;
 
     node2y = Dict{Int, VariableRef}()
     for (layer, nodes) in enumerate(layer2nodes)
+        isempty(nodes) && continue # skip empty layers
         first_node, other_nodes = Iterators.peel(nodes)
         prev_y = node2y[first_node] = @variable(m, base_name="y_$first_node")
         for node in other_nodes
diff --git a/test/demos.jl b/test/demos.jl
index 281030a..af63b63 100644
--- a/test/demos.jl
+++ b/test/demos.jl
@@ -78,4 +78,17 @@ end
     test_example(layout, :two_lines_flipped_vertex_order; force_equal_layers=[1=>3])
     #test_example(layout, :large_depgraph)  # too big
     #test_example(layout, :extra_large_depgraph)  # too big
+
+    # laying out a graph with specified layering values - which intentionally skips the 16th
+    # layer entirely - while also using `force_equal_layers` relative to this layering set
+    test_example(layout, :disconnected_components_graph;
+        force_layer = [ # note that the 16th layer is skipped
+            1=>1, 2=>1, 3=>2, 4=>2, 5=>3, 6=>4, 7=>4, 8=>5, 9=>6, 10=>6, 11=>7, 12=>7, 
+            13=>8, 14=>8, 15=>9, 16=>9, 17=>10, 18=>10, 19=>11, 20=>12, 21=>12, 22=>13,
+            23=>14, 24=>14, 25=>15, 26=>15, 27=>17, 28=>18, 29=>18, 30=>19, 31=>19, 32=>20,
+            33=>20, 34=>21, 35=>22, 36=>22, 37=>23, 38=>24, 39=>24, 40=>25, 41=>25, 42=>26,
+            43=>26, 44=>27, 45=>27, 46=>28, 47=>28, 48=>29, 49=>30, 50=>30, 51=>31, 52=>31],
+        force_equal_layers = [
+            1=>7, 1=>9, 1=>15, 3=>5, 3=>11, 3=>13, 17=>21, 17=>23, 17=>29, 19=>25, 19=>27,
+            19=>31])
 end
diff --git a/test/examples.jl b/test/examples.jl
index 4f430f9..90d29e3 100644
--- a/test/examples.jl
+++ b/test/examples.jl
@@ -488,6 +488,66 @@ module Examples
         2 => 7
         2 => 24
     ]))
+
+    # a graph with disconnected components
+    disconnected_components_graph = SimpleDiGraph(Edge.([
+        1 => 3
+        2 => 4
+        3 => 5
+        4 => 5
+        5 => 6
+        5 => 7
+        6 => 8
+        7 => 8
+        8 => 9
+        8 => 10
+        9 => 11
+        10 => 12
+        11 => 13
+        12 => 14
+        13 => 16
+        14 => 15
+        15 => 18
+        16 => 17
+        17 => 19
+        18 => 19
+        19 => 20
+        19 => 21
+        20 => 22
+        21 => 22
+        22 => 23
+        22 => 24
+        23 => 26
+        24 => 25
+        27 => 28
+        27 => 29
+        28 => 30
+        29 => 31
+        30 => 32
+        31 => 33
+        32 => 34
+        33 => 34
+        34 => 35
+        34 => 36
+        35 => 37
+        36 => 37
+        37 => 38
+        37 => 39
+        38 => 41
+        39 => 40
+        40 => 43
+        41 => 42
+        42 => 44
+        43 => 45
+        44 => 46
+        45 => 47
+        46 => 48
+        47 => 48
+        48 => 49
+        48 => 50
+        49 => 52
+        50 => 51
+    ]))
 end  # module
 
 
diff --git a/test/references/Zarate/direct/disconnected_components_graph_force_layer_force_equal_layers.png b/test/references/Zarate/direct/disconnected_components_graph_force_layer_force_equal_layers.png
new file mode 100644
index 0000000..0a6cbb2
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diff --git a/test/references/Zarate/paths/disconnected_components_graph_force_layer_force_equal_layers.png b/test/references/Zarate/paths/disconnected_components_graph_force_layer_force_equal_layers.png
new file mode 100644
index 0000000..0a6cbb2
Binary files /dev/null and b/test/references/Zarate/paths/disconnected_components_graph_force_layer_force_equal_layers.png differ