diff --git a/build-caffeine-exe.ps1 b/build-caffeine-exe.ps1
index f6838a8..58ff385 100644
--- a/build-caffeine-exe.ps1
+++ b/build-caffeine-exe.ps1
@@ -1,8 +1,8 @@
 [CmdletBinding()]
 param(
-    [string]$SourceScript = (Join-Path $PSScriptRoot 'caffeine.ps1'),
-    [string]$OutputDir = (Join-Path $PSScriptRoot 'dist'),
-    [string]$BuildDir = (Join-Path $PSScriptRoot 'build'),
+    [string]$SourceScript,
+    [string]$OutputDir,
+    [string]$BuildDir,
     [string]$ExeName = 'PortableCaffeine.exe',
     [switch]$ConsoleTarget,
     [switch]$KeepExtractedSource
@@ -11,6 +11,26 @@ param(
 Set-StrictMode -Version Latest
 $ErrorActionPreference = 'Stop'
 
+# In Windows PowerShell 5.1, $PSScriptRoot may be empty during param default evaluation.
+# Resolve script-relative defaults after the param block for compatibility.
+$scriptRoot = if (-not [string]::IsNullOrWhiteSpace($PSScriptRoot)) {
+    $PSScriptRoot
+} elseif ($MyInvocation.MyCommand.Path) {
+    Split-Path -Parent $MyInvocation.MyCommand.Path
+} else {
+    (Get-Location).Path
+}
+
+if ([string]::IsNullOrWhiteSpace($SourceScript)) {
+    $SourceScript = Join-Path $scriptRoot 'caffeine.ps1'
+}
+if ([string]::IsNullOrWhiteSpace($OutputDir)) {
+    $OutputDir = Join-Path $scriptRoot 'dist'
+}
+if ([string]::IsNullOrWhiteSpace($BuildDir)) {
+    $BuildDir = Join-Path $scriptRoot 'build'
+}
+
 function Get-CSharpFromCaffeineScript {
     param(
         [Parameter(Mandatory = $true)]
diff --git a/sign-caffeine-exe.ps1 b/sign-caffeine-exe.ps1
index b62ccf9..7ae85a2 100644
--- a/sign-caffeine-exe.ps1
+++ b/sign-caffeine-exe.ps1
@@ -1,6 +1,6 @@
 [CmdletBinding()]
 param(
-    [string]$ExePath = (Join-Path $PSScriptRoot 'dist\PortableCaffeine.exe'),
+    [string]$ExePath,
     [string]$Subject = 'CN=Portable Caffeine Dev Code Signing',
     [string]$FriendlyName = 'Portable Caffeine Dev Code Signing (Self-Signed)',
     [ValidateSet('None', 'CurrentUser', 'LocalMachine')]
@@ -10,7 +10,7 @@ param(
     [int]$YearsValid = 5,
     [switch]$ForceNewCertificate,
     [string]$TimestampServer,
-    [string]$CerExportPath = (Join-Path $PSScriptRoot 'dist\PortableCaffeine-dev-signing.cer'),
+    [string]$CerExportPath,
     [string]$PfxExportPath,
     [securestring]$PfxPassword
 )
@@ -18,6 +18,23 @@ param(
 Set-StrictMode -Version Latest
 $ErrorActionPreference = 'Stop'
 
+# In Windows PowerShell 5.1, $PSScriptRoot may be empty during param default evaluation.
+# Resolve script-relative defaults after the param block for compatibility.
+$scriptRoot = if (-not [string]::IsNullOrWhiteSpace($PSScriptRoot)) {
+    $PSScriptRoot
+} elseif ($MyInvocation.MyCommand.Path) {
+    Split-Path -Parent $MyInvocation.MyCommand.Path
+} else {
+    (Get-Location).Path
+}
+
+if ([string]::IsNullOrWhiteSpace($ExePath)) {
+    $ExePath = Join-Path $scriptRoot 'dist\PortableCaffeine.exe'
+}
+if ([string]::IsNullOrWhiteSpace($CerExportPath)) {
+    $CerExportPath = Join-Path $scriptRoot 'dist\PortableCaffeine-dev-signing.cer'
+}
+
 if ([Environment]::OSVersion.Platform -ne [PlatformID]::Win32NT) {
     throw "This script is intended for Windows only."
 }
diff --git a/tasks/lessons.md b/tasks/lessons.md
new file mode 100644
index 0000000..b300d57
--- /dev/null
+++ b/tasks/lessons.md
@@ -0,0 +1,5 @@
+# Lessons
+
+## 2026-02-24
+
+- PowerShell 5.1 compatibility: do not use `$PSScriptRoot` (or other script-location variables) inside `param()` default expressions. Resolve script-relative defaults after the `param` block instead.