diff --git a/CHANGELOG.md b/CHANGELOG.md
index a5180a8..2b85ded 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,9 +1,11 @@
-### 0.1.6 (Upcoming release) 
+### 0.1.7 (Upcoming release) 
+
+### 0.1.6 
 
 - Add Latex support for Transportation tables
 - Pretty printing simplex iterations
 - Automatic calculation of objective value by iterations and manual calculation code is removed
-
+- Non-negativity conditions of models in the definition stage
 
 ### 0.1.5 
 
diff --git a/Project.toml b/Project.toml
index 89624f4..4622c2a 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "OperationsResearchModels"
 uuid = "8042aa49-e599-49ec-a8f7-b5b80cc82a88"
 authors = ["Mehmet Hakan Satman <mhsatman@gmail.com>"]
-version = "0.1.5"
+version = "0.1.6"
 
 [deps]
 HiGHS = "87dc4568-4c63-4d18-b0c0-bb2238e4078b"
diff --git a/src/maximumflow.jl b/src/maximumflow.jl
index d0e9b54..abb2f0e 100644
--- a/src/maximumflow.jl
+++ b/src/maximumflow.jl
@@ -40,7 +40,7 @@ function solve(cns::Array{Connection,1})
             return :f
         end
         expr = @expression(model, 0)
-        for i = 1:length(lst)
+        for i = eachindex(lst)
             expr += x[lst[i].from, lst[i].to]
         end
         return expr
@@ -58,7 +58,7 @@ function solve(cns::Array{Connection,1})
 
     # Variables 
     @variable(model, f)
-    @variable(model, x[1:n, 1:n])
+    @variable(model, x[1:n, 1:n] .>= 0)
 
     # Objective Function
     @objective(model, Max, f)
@@ -78,7 +78,6 @@ function solve(cns::Array{Connection,1})
 
     for nd in cns
         @constraint(model, x[nd.from, nd.to] <= nd.value)
-        @constraint(model, x[nd.from, nd.to] >= 0)
     end
 
     @constraint(model, f >= 0)
diff --git a/src/shortestpath.jl b/src/shortestpath.jl
index 48490aa..a88f398 100644
--- a/src/shortestpath.jl
+++ b/src/shortestpath.jl
@@ -41,7 +41,7 @@ function solve(cns::Array{Connection,1})
             return 0
         end
         expr = @expression(model, 0)
-        for i = 1:length(lst)
+        for i = eachindex(lst)
             expr += x[lst[i].from, lst[i].to]
         end
         return expr
diff --git a/src/transportation.jl b/src/transportation.jl
index 905c9c1..e47077e 100644
--- a/src/transportation.jl
+++ b/src/transportation.jl
@@ -85,12 +85,11 @@ function solve(t::TransportationProblem)::TransportationResult
 
     n, p = size(newt.costs)
 
-    @variable(model, x[1:n, 1:p])
+    @variable(model, x[1:n, 1:p] .>= 0)
     @objective(model, Min, sum(newt.costs .* x))
 
     @constraint(model, sum(x[1:n, j] for j = 1:p) .== newt.supply)
     @constraint(model, sum(x[i, 1:p] for i = 1:n) .== newt.demand)
-    @constraint(model, x .>= 0)
 
     optimize!(model)