diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 32d75567..22f7efcc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,6 +15,8 @@ jobs:
       fail-fast: false
       matrix:
         version:
+          - '1.5'
+          - '1.4'
           - '1.3' # Replace this with the minimum Julia version that your package supports. E.g. if your package requires Julia 1.5 or higher, change this to '1.5'.
           - '1' # Leave this line unchanged. '1' will automatically expand to the latest stable 1.x release of Julia.
           - 'nightly'
@@ -40,6 +42,8 @@ jobs:
             ${{ runner.os }}-
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
+        env:
+          JULIA_NUM_THREADS: "3"
       - uses: julia-actions/julia-processcoverage@v1
       - uses: codecov/codecov-action@v1
         with:
@@ -67,4 +71,3 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}
-          JULIA_NUM_THREADS: "3"
diff --git a/Project.toml b/Project.toml
index 480633b7..464e5be4 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "UnROOT"
 uuid = "3cd96dde-e98d-4713-81e9-a4a1b0235ce9"
 authors = ["Tamas Gal", "Jerry Ling", "Johannes Schumann", "Nick Amin"]
-version = "0.3.7"
+version = "0.4.0"
 
 [deps]
 AbstractTrees = "1520ce14-60c1-5f80-bbc7-55ef81b5835c"
diff --git a/src/UnROOT.jl b/src/UnROOT.jl
index f2b242a7..63f9b60a 100644
--- a/src/UnROOT.jl
+++ b/src/UnROOT.jl
@@ -12,8 +12,9 @@ using Mixers, Parameters, Memoization, LRUCache
 
 import Tables, TypedTables, PrettyTables, DataFrames
 
-@static if VERSION < v"1.4"
+@static if VERSION < v"1.6"
     Base.first(a::AbstractVector{S}, n::Integer) where S<: AbstractString = a[1:(length(a) > n ? n : end)]
+    Base.first(a::S, n::Integer) where S<: AbstractString = a[1:(length(a) > n ? n : end)]
 end
 
 include("constants.jl")
diff --git a/src/iteration.jl b/src/iteration.jl
index c4395a3d..fce82358 100644
--- a/src/iteration.jl
+++ b/src/iteration.jl
@@ -101,7 +101,7 @@ mutable struct LazyBranch{T,J,B} <: AbstractVector{T}
         return new{T,J,typeof(_buffer)}(f, b, length(b),
                                         b.fBasketEntry,
                                         [_buffer for _ in 1:Threads.nthreads()],
-                                        [0:0 for _ in 1:Threads.nthreads()])
+                                        [0:-1 for _ in 1:Threads.nthreads()])
     end
 end
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 0e81e024..534e3c4f 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -464,7 +464,7 @@ end
     rootfile = ROOTFile(joinpath(SAMPLES_DIR, "issue61.root"))
     arr = LazyTree(rootfile,"Events").Jet_pt;
     _ = length.(arr);
-    @test length(arr.buffer[1]) == length(arr.buffer_range[1])
+    @test length.(arr.buffer) == length.(arr.buffer_range)
     close(rootfile)
 end
 
@@ -522,6 +522,9 @@ end
     @test nmu == 878
 
 
+    if get(ENV, "CI", "false") == "true"
+        @test Threads.nthreads() > 1
+    end
     nmus = zeros(Int, Threads.nthreads())
     Threads.@threads for i in 1:length(t)
         nmus[Threads.threadid()] += length(t.Muon_pt[i])