diff --git a/src/api/types.jl b/src/api/types.jl
index 5c82e17..e37ecf7 100644
--- a/src/api/types.jl
+++ b/src/api/types.jl
@@ -171,7 +171,10 @@ abstract type AbstractStrategy end
 
 """Empty evolution strategy"""
 struct NoStrategy <: AbstractStrategy end
-copy(s::NoStrategy) = NoStrategy()
+copy(::NoStrategy) = NoStrategy()
+Base.getproperty(s::NoStrategy, ::Symbol) = 0.0
+Base.setproperty!(s::NoStrategy, ::Symbol, ::T) where {T<:Real} = 0.0
+
 
 """
 Isotropic evolution strategy
diff --git a/src/es.jl b/src/es.jl
index 2616992..65849b0 100644
--- a/src/es.jl
+++ b/src/es.jl
@@ -4,8 +4,8 @@ Implementation of Evolution Strategy: (μ/ρ(+/,)λ)-ES
 The constructor takes following keyword arguments:
 
 - `initStrategy`: an initial strategy description, (default: empty)
-- `recombination`: ES recombination function for population (default: `first`), see [Crossover](@ref)
-- `srecombination`: ES recombination function for strategies (default: `first`), see [Crossover](@ref)
+- `recombination`: ES recombination function for population (default: [`average`](@ref)), see [Crossover](@ref)
+- `srecombination`: ES recombination function for strategies (default: [`average`](@ref)), see [Crossover](@ref)
 - `mutation`: [Mutation](@ref) function for population (default: [`nop`](@ref))
 - `smutation`: [Mutation](@ref) function for strategies (default: [`nop`](@ref))
 - `μ`/`mu`: the number of parents
@@ -27,8 +27,8 @@ struct ES{T1,T2,T3,T4} <: AbstractOptimizer
     metrics::ConvergenceMetrics
 
     ES(; initStrategy::AbstractStrategy = NoStrategy(),
-        recombination::T1 = first,
-        srecombination::T2 = first,
+        recombination::T1 = average,
+        srecombination::T2 = average,
         mutation::T3 = nop,
         smutation::T4 = nop,
         μ::Integer = 1,
diff --git a/src/mutations.jl b/src/mutations.jl
index ec4ae2a..20aefa0 100644
--- a/src/mutations.jl
+++ b/src/mutations.jl
@@ -5,14 +5,14 @@
 # ======================
 
 """
-    nop(s::AbstractStrategy)
+    nop(x, s::AbstractStrategy; kwargs...)
 
-This is a dummy mutation operator that does not change the strategy.
+This is a dummy mutation operator of the recombinant `x` that does not change the strategy `s`.
 """
 nop(recombinant::AbstractVector, s::AbstractStrategy; kwargs...) = recombinant
 
 """
-    gaussian(x, s::IsotropicStrategy)
+    gaussian(x, s::IsotropicStrategy; rng=Random.default_rng())
 
 Performs Gaussian isotropic mutation of the recombinant `x` given the strategy `s`  by adding Gaussian noise as follows:
 
@@ -27,7 +27,7 @@ function gaussian(recombinant::AbstractVector, s::IsotropicStrategy;
 end
 
 """
-    gaussian(x, s::AnisotropicStrategy)
+    gaussian(x, s::AnisotropicStrategy; rng=Random.default_rng())
 
 Performs Gaussian anisotropic mutation of the recombinant `x` given the strategy `s`  by adding Gaussian noise as follows:
 
@@ -43,7 +43,7 @@ function gaussian(recombinant::AbstractVector, s::AnisotropicStrategy;
 end
 
 """
-    cauchy(x, s::IsotropicStrategy)
+    cauchy(x, s::IsotropicStrategy; rng=Random.default_rng())
 
 Performs isotropic mutation of the recombinant `x` given the strategy `s`  by adding a noise from the Cauchy distribution as follows:
 
@@ -65,7 +65,7 @@ end
 # ===========================
 
 """
-    nop(s::AbstractStrategy)
+    nop(s::AbstractStrategy; kwargs...)
 
 This is a dummy operator that does not change strategy.
 """
@@ -73,7 +73,7 @@ nop(s::AbstractStrategy; kwargs...) = s
 
 
 """
-    gaussian(s::IsotropicStrategy)
+    gaussian(s::IsotropicStrategy; rng=Random.default_rng())
 
 Performs in-place mutation of the isotropic strategy `s` modifying its mutated strategy parameter ``\\sigma`` with Gaussian noise as follows:
 
@@ -87,7 +87,7 @@ end
 
 
 """
-    gaussian(s::AnisotropicStrategy)
+    gaussian(s::AnisotropicStrategy; rng=Random.default_rng())
 
 Performs in-place mutation of the anisotropic strategy `s` modifying its mutated strategy parameter ``\\sigma`` with Gaussian noise as follows:
 
@@ -107,14 +107,14 @@ end
 # ---------------------
 
 """
-    genop(recombinant)
+    genop(recombinant; kwargs...)
 
 This is a dummy mutation operator that does not change the `recombinant`.
 """
 genop(recombinant; kwargs...) = recombinant
 
 """
-    flip(recombinant)
+    flip(recombinant; rng=Random.default_rng())
 
 Returns an in-place mutated binary `recombinant` with a bit flips at random positions.
 """
@@ -131,7 +131,7 @@ end
 
 Returns an in-place mutated binary `recombinant` with its bits inverted.
 """
-bitinversion(recombinant::T) where {T <: AbstractVector{Bool}} = map!(!, recombinant, recombinant)
+bitinversion(recombinant::T; kwargs...) where {T <: AbstractVector{Bool}} = map!(!, recombinant, recombinant)
 
 
 # Real-valued mutations
diff --git a/src/recombinations.jl b/src/recombinations.jl
index 8ec65a6..f4f932a 100644
--- a/src/recombinations.jl
+++ b/src/recombinations.jl
@@ -1,7 +1,7 @@
 # Recombinations
 # ==============
 """
-    average(population)
+    average(population; kwargs...)
 
 Returns an *one* offspring individual of a multi-parent recombination by averaging `population`.
 """
@@ -15,7 +15,7 @@ function average(population::Vector{T}; kwargs...) where {T <: AbstractVector}
 end
 
 """
-    marriage(population)
+    marriage(population; rng=Radom.default_rng())
 
 Returns an *one* offspring individual of a multi-parent recombination by random copying from `population`.
 """
@@ -38,7 +38,7 @@ end
 
 Returns the average value of the mutation parameter ``\\sigma`` of strategies `ss`.
 """
-function average(ss::Vector{<:AbstractStrategy}; kwargs...)
+function average(ss::Vector{<:AbstractStrategy})
     s = copy(first(ss))
     l = length(ss)
     s.σ = mapreduce(s->s.σ/l, +, ss)
diff --git a/test/issues.jl b/test/issues.jl
new file mode 100644
index 0000000..3bb0ab5
--- /dev/null
+++ b/test/issues.jl
@@ -0,0 +1,6 @@
+@testset "Issues" begin
+    # issue #105
+    res = Evolutionary.optimize(sum, ones(5), ES(μ = 40, λ = 100))
+    @test minimum(res) == 5
+
+end
\ No newline at end of file
diff --git a/test/runtests.jl b/test/runtests.jl
index c2da747..9f87363 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -24,7 +24,8 @@ for tests in [
     "onemax.jl",
     "moea.jl",
     "regression.jl",
-    "gp.jl"
+    "gp.jl",
+    "issues.jl"
 ]
     include(tests)
 end