diff --git a/src/operators.jl b/src/operators.jl
index 77496d323..aa788553e 100644
--- a/src/operators.jl
+++ b/src/operators.jl
@@ -712,7 +712,7 @@ Determine how many elements of `x` are less than `i` for all `i` in `1:n`.
 """
 function compute_shifts(n::Integer, x::AbstractArray)
     tmp = zeros(eltype(x), n)
-    tmp[x[2:end]] .= 1
+    tmp[x] .= 1
     return cumsum!(tmp, tmp)
 end
 
@@ -743,29 +743,25 @@ julia> collect(edges(h))
  Edge 3 => 4
 ```
 """
-function merge_vertices(g::AbstractGraph, vs)
-    labels = collect(1:nv(g))
+function merge_vertices(g::AbstractSimpleGraph, vs)
     # Use lowest value as new vertex id.
-    sort!(vs)
-    nvnew = nv(g) - length(unique(vs)) + 1
+    vs = unique!(sort(vs))
+    merged_vertex = popfirst!(vs)
+
+    nvnew = nv(g) - length(vs)
     nvnew <= nv(g) || return g
-    (v0, vm) = extrema(vs)
-    v0 > 0 || throw(ArgumentError("invalid vertex ID: $v0 in list of vertices to be merged"))
-    vm <= nv(g) || throw(ArgumentError("vertex $vm not found in graph")) # TODO 0.7: change to DomainError?
-    labels[vs] .= v0
-    shifts = compute_shifts(nv(g), vs[2:end])
-    for v in vertices(g)
-        if labels[v] != v0
-            labels[v] -= shifts[v]
-        end
-    end
+    merged_vertex > 0 || throw(ArgumentError("invalid vertex ID: $merged_vertex in list of vertices to be merged"))
+    vs[end] <= nv(g) || throw(ArgumentError("vertex $(vs[end]) not found in graph")) # TODO 0.7: change to DomainError?
+
+    new_vertex_ids = collect(vertices(g)) .- compute_shifts(nv(g), vs)
+    new_vertex_ids[vs] .= merged_vertex
 
     #if v in vs then labels[v] == v0 else labels[v] == v
     newg = SimpleGraph(nvnew)
     for e in edges(g)
         u, w = src(e), dst(e)
-        if labels[u] != labels[w] #not a new self loop
-            add_edge!(newg, labels[u], labels[w])
+        if new_vertex_ids[u] != new_vertex_ids[w] #not a new self loop
+            add_edge!(newg, new_vertex_ids[u], new_vertex_ids[w])
         end
     end
     return newg
@@ -812,35 +808,27 @@ julia> collect(edges(g))
 ```
 """
 function merge_vertices!(g::Graph{T}, vs::Vector{U} where U <: Integer) where T
-    vs = sort!(unique(vs))
-    merged_vertex = popfirst!(vs)
+    vs = unique!(sort(vs))
+    (merged_vertex, vm) = extrema(vs)
 
-    x = zeros(Int, nv(g))
-    x[vs] .= 1
-    new_vertex_ids = collect(1:nv(g)) .- cumsum(x)
+    merged_vertex > 0 || throw(ArgumentError("invalid vertex ID: $merged_vertex in list of vertices to be merged"))
+    vm <= nv(g) || throw(ArgumentError("vertex $vm not found in graph")) # TODO 0.7: change to DomainError?
+
+    new_vertex_ids = collect(vertices(g)) .- compute_shifts(nv(g), vs[2:end])
     new_vertex_ids[vs] .= merged_vertex
 
     for i in vertices(g)
         # Adjust connections to merged vertices
-        if (i != merged_vertex) && !insorted(i, vs)
+        if new_vertex_ids[i] != merged_vertex
             nbrs_to_rewire = Set{T}()
             for j in outneighbors(g, i)
-                if insorted(j, vs)
-                    push!(nbrs_to_rewire, merged_vertex)
-                else
-                    push!(nbrs_to_rewire, new_vertex_ids[j])
-                end
+                push!(nbrs_to_rewire, new_vertex_ids[j])
             end
-            g.fadjlist[new_vertex_ids[i]] = sort(collect(nbrs_to_rewire))
-
+            g.fadjlist[new_vertex_ids[i]] = sort!(collect(nbrs_to_rewire))
 
         # Collect connections to new merged vertex
         else
             nbrs_to_merge = Set{T}()
-            for element in filter(x -> !(insorted(x, vs)) && (x != merged_vertex), g.fadjlist[i])
-                push!(nbrs_to_merge, new_vertex_ids[element])
-            end
-
             for j in vs, e in outneighbors(g, j)
                 if new_vertex_ids[e] != merged_vertex
                     push!(nbrs_to_merge, new_vertex_ids[e])
@@ -850,8 +838,9 @@ function merge_vertices!(g::Graph{T}, vs::Vector{U} where U <: Integer) where T
         end
     end
 
+
     # Drop excess vertices
-    g.fadjlist = g.fadjlist[1:(end - length(vs))]
+    g.fadjlist = g.fadjlist[begin:(end - length(vs)+1)]
 
     # Correct edge counts
     g.ne = sum(degree(g, i) for i in vertices(g)) / 2
diff --git a/test/operators.jl b/test/operators.jl
index a0809205f..39fa97963 100644
--- a/test/operators.jl
+++ b/test/operators.jl
@@ -103,6 +103,13 @@
             @test neighbors(h2, 5) == [2]
             @test ne(h2) == 3
             @test nv(h2) == 5
+
+            h3 = star_graph(5)
+            h3merged = merge_vertices(h3, [1,2])
+            @test neighbors(h3merged, 1) == [2,3,4]
+            @test neighbors(h3merged, 2) == [1]
+            @test neighbors(h3merged, 3) == [1]
+            @test neighbors(h3merged, 4) == [1]
         end
     end