diff --git a/Export-Excel.Tests.ps1 b/Export-Excel.Tests.ps1
index b6f7ecf..c59ae7f 100644
--- a/Export-Excel.Tests.ps1
+++ b/Export-Excel.Tests.ps1
@@ -1,64 +1,94 @@
-# Contributed by https://github.com/W1M0R
+#Requires -Modules Pester
+#Requires -Modules Assert
 
-Import-Module ImportExcel -Force 
+$here = Split-Path -Parent $MyInvocation.MyCommand.Path
+$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path) -replace '\.Tests\.', '.'
 
-function New-TestWorkbook {
-    $testWorkbook = "$($PSScriptRoot)\test.xlsx"
-    
-    Remove-Item $testWorkbook -ErrorAction Ignore
-    $testWorkbook
+Import-Module $here -Force
+
+$WarningPreference = 'SilentlyContinue'
+$ProgressPreference = 'SilentlyContinue'
+
+Function Test-isNumeric {
+    Param (
+        [Parameter(ValueFromPipeline)]$x
+    )
+
+    Return $x -is [byte]  -or $x -is [int16]  -or $x -is [int32]  -or $x -is [int64]  `
+       -or $x -is [sbyte] -or $x -is [uint16] -or $x -is [uint32] -or $x -is [uint64] `
+       -or $x -is [float] -or $x -is [double] -or $x -is [decimal]
 }
 
-function Remove-TestWorkbook {
-    New-TestWorkbook | Out-Null
+$fakeData = [PSCustOmobject]@{
+    Property_1_Date      = (Get-Date).ToString('d') # US '10/16/2017' BE '16/10/2107'
+    Property_2_Formula   = '=SUM(G2:H2)'
+    Property_3_String    = 'My String'
+    Property_4_String    = 'a'
+    Property_5_IPAddress = '10.10.25.5'
+    Property_6_Number    = '0'
+    Property_7_Number    = '5'
+    Property_8_Number    = '007'
+    Property_9_Number    = (33).ToString('F2') # US '33.00' BE '33,00'
+    Property_10_Number   = (5/3).ToString('F2') # US '1.67' BE '1,67'
+    Property_11_Number   = (15999998/3).ToString('N2') # US '5,333,332.67' BE '5.333.332,67'
+    Property_12_Number   = '1.555,83'
+    Property_13_PhoneNr  = '+32 44' 
+    Property_14_PhoneNr  = '+32 4 4444 444'
+    Property_15_PhoneNr  =  '+3244444444'
 }
 
-function New-TestDataCsv {
-@"
-ID,Product,Quantity,Price,Total
-12001,Nails,37,3.99,147.63
-12002,Hammer,5,12.10,60.5
-12003,Saw,12,15.37,184.44
-01200,Drill,20,8,160  
-00120,Crowbar,7,23.48,164.36
-true,Bla,7,82,12
-false,Bla,7,82,12
-2009-05-01 14:57:32.8,Yay,1,3,2
-"@ | ConvertFrom-Csv 
-}
+$Path = 'Test.xlsx'
 
-Describe "Export-Excel" {
+Describe 'Export-Excel' {
+    in $TestDrive {
+        Describe 'Number conversion' {
+            Context 'numerical values expected' {
+                #region Create test file                
+                $fakeData | Export-Excel -Path $Path
+                
+                $Path = $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($Path)
+                $Excel = New-Object OfficeOpenXml.ExcelPackage $Path
+                $Worksheet = $Excel.Workbook.WorkSheets[1]
+                #endregion
 
-    $csvData  = New-TestDataCsv
-    $workbook = New-TestWorkbook
+                it 'zero' {
+                    $fakeData.Property_6_Number | Should -BeExactly '0'
+                    $Worksheet.Cells[2, 6].Text | Should -BeExactly $fakeData.Property_6_Number
+                    $Worksheet.Cells[2, 6].Value | Test-isNumeric | Should -Be $true
+                }
 
-    Context "Importing CSV data from a here string" {
-        It "All properties are type [string]" {
-            $csvData | % {
-                $_.PSObject.Properties | % {
-                    $_.Value -is [string] | Should Be $true
+                It 'regular number' {
+                    $fakeData.Property_7_Number | Should -BeExactly '5'
+                    $Worksheet.Cells[2, 7].Text | Should -BeExactly $fakeData.Property_7_Number
+                    $Worksheet.Cells[2, 7].Value | Test-isNumeric | Should -Be $true
+                }
+                
+                It 'number starting with zero' {
+                    $fakeData.Property_8_Number | Should -BeExactly '007'
+                    $Worksheet.Cells[2, 8].Text | Should -BeExactly '7'
+                    $Worksheet.Cells[2, 8].Value | Test-isNumeric | Should -Be $true
+                }
+                
+                It 'decimal number' {
+                    # US '33.00' BE '33,00'
+                    $fakeData.Property_9_Number | Should -BeExactly (33).ToString('F2')
+                    $Worksheet.Cells[2, 9].Text | Should -BeExactly '33'
+                    $Worksheet.Cells[2, 9].Value | Test-isNumeric | Should -Be $true
+
+                    # US '1.67' BE '1,67'
+                    $fakeData.Property_10_Number | Should -BeExactly (5/3).ToString('F2')
+                    $Worksheet.Cells[2, 10].Text | Should -BeExactly $fakeData.Property_10_Number
+                    $Worksheet.Cells[2, 10].Value | Test-isNumeric | Should -Be $true
+                }
+
+                It 'thousand seperator and decimal number' {
+                    # US '5,333,332.67' BE '5.333.332,67'
+                    # Excel BE '5333332,67'
+                    $fakeData.Property_11_Number | Should -BeExactly (15999998/3).ToString('N2')
+                    $Worksheet.Cells[2, 11].Text | Should -BeExactly $fakeData.Property_11_Number
+                    $Worksheet.Cells[2, 11].Value | Test-isNumeric | Should -Be $true
                 }
             }
         }
-        It "Leading zeroes are preserved" {
-            $csvData[4] | Select-Object -ExpandProperty ID | Should Be "00120"
-        }
     }
-
-    Context "Piping CSV data to Export-Excel" {
-
-        $xlPkg = $csvData | Export-Excel $workbook -PassThru
-        $ws = $xlPkg.Workbook.WorkSheets[1]
-
-        It "Exports numeric strings as numbers" {
-            $csvData[2] | Select-Object -ExpandProperty ID | Should Be "12003"
-            $ws.Cells["A4"].Value -is [double] | Should Be $true
-            $ws.Cells["A4"].Value | Should Be 12003
-        }
-
-        $xlPkg.Save()
-        $xlPkg.Dispose()
-    }
-
-    Remove-TestWorkbook
 }
\ No newline at end of file
diff --git a/Export-Excel.ps1 b/Export-Excel.ps1
index 96e4faf..516b07b 100644
--- a/Export-Excel.ps1
+++ b/Export-Excel.ps1
@@ -310,7 +310,7 @@ Function Export-Excel {
 
                 Default {
                     #region Save a value as a number if possible
-                    if ($Number = ConvertTo-Number $_) {
+                    if (($Number = ConvertTo-Number $_) -ne $null) {
                         $TargetCell.Value = $Number
                         $targetCell.Style.Numberformat.Format = $Numberformat
                         Write-Verbose "Cell '$Row`:$ColumnIndex' header '$Name' add value '$($TargetCell.Value)' as number converted from '$_' with format '$Numberformat'"