fix: clear error when rule Condition is null

CHANGELOG.md

@@ -18,6 +18,12 @@
   bind and always threw). `-PropertySet` is now a mandatory parameter on
   `Test-FeatureFlag`, matching `Test-Condition`, so the missing value is
   reported with a clear mandatory-parameter error instead. (#84)
+- `Test-Condition`/`Test-FeatureFlag` no longer throw an opaque
+  "You cannot call a method on a null-valued expression" error when a rule's
+  `Condition` is `$null` (e.g. omitted or misspelled). `ConditionTransformAttribute`
+  now null-guards and raises a clear `ArgumentNullException` explaining that the
+  rule must define a `Condition`. `FeatureFlagTransformAttribute` gained the same
+  guard for a null `FeatureFlag`. (#83)
 
 ## [1.0.0] 2026-06-05
 
@@ -78,7 +84,7 @@
   constraints now correctly returns `$false` instead of continuing
   evaluation.
 - `PropertyValidation` threshold fields (`Minimum`, `Maximum`,
   `MinLength`, `MaxLength`) changed to `Nullable[int]` so an unconfigured
   constraint is no longer treated as zero.
 - `PropertyValidation.ToHashtable` now omits null/unset fields to
   prevent invalid entries in serialized JSON during Save/FromFile
@@ -149,7 +155,7 @@
 ### Changed
 
 - `Test-Condition` function now accepts `ConditionGroup` objects instead of
   hashtables for the Condition parameter, providing stronger type safety.
 - `Test-Condition` function now accepts `PropertySet` objects instead of a
   generic Properties hashtable for the Properties parameter.
 - Internal condition evaluation logic updated to use null checks on

Gatekeeper/Classes/FeatureFlag.ps1

@@ -236,6 +236,10 @@ class GatekeeperPath {
 class FeatureFlagTransformAttribute : System.Management.Automation.ArgumentTransformationAttribute {
 
     [object] Transform([System.Management.Automation.EngineIntrinsics]$engineIntrinsics, [object] $inputData) {
+        if ($null -eq $inputData) {
+            throw [System.ArgumentNullException]::new('FeatureFlag',
+                "FeatureFlag cannot be null.")
+        }
         if ($inputData -is [FeatureFlag]) {
             return $inputData
         }
@@ -258,6 +262,10 @@ class FeatureFlagTransformAttribute : System.Management.Automation.ArgumentTrans
 class ConditionTransformAttribute : System.Management.Automation.ArgumentTransformationAttribute {
 
     [object] Transform([System.Management.Automation.EngineIntrinsics]$engineIntrinsics, [object] $inputData) {
+        if ($null -eq $inputData) {
+            throw [System.ArgumentNullException]::new('Condition',
+                "Rule has no Condition. Each rule must define a 'Condition'.")
+        }
         if ($inputData -is [Condition]) {
             return $inputData
         }

tests/Classes.tests.ps1

@@ -216,6 +216,12 @@
             Test-Condition -Condition 42 -PropertySet $script:propertySet -Context $script:context
         } | Should -Throw -ExpectedMessage '*Cannot convert type*'
     }
+
+    It 'Throws a clear, actionable error when Condition is null' {
+        {
+            Test-Condition -Condition $null -PropertySet $script:propertySet -Context $script:context
+        } | Should -Throw -ExpectedMessage "*Rule has no Condition*"
+    }
 }
 
 Describe 'PropertySetTransformAttribute' {
@@ -323,7 +329,7 @@
             Type       = 'string'
             Validation = @{ MinLength = 1; MaxLength = 5 }
         })
         $pd.Validate('toolongstring') | Should -BeFalse
     }
 
     It 'Returns $false when string does not match pattern' {

tests/Test-FeatureFlag.tests.ps1

@@ -85,6 +85,18 @@ Describe 'Test-FeatureFlag' {
         }
     }
 
+    Context 'Malformed rules' {
+        It 'Rule with a null Condition throws a clear, actionable error' {
+            $flag = [FeatureFlag]::new(@{
+                Name = 'MissingConditionFlag'; DefaultEffect = 'Deny'; Version = '1.0.0'; Author = 'Test'
+                Rules = @(@{ Name = 'Allow group'; Effect = 'Allow' })
+            })
+            {
+                Test-FeatureFlag -FeatureFlag $flag -PropertySet $script:propertySet -Context $script:context
+            } | Should -Throw -ExpectedMessage '*Rule has no Condition*'
+        }
+    }
+
     Context 'Ordering — first-match-wins' {
         It 'Deny before matching Allow returns $false (Deny wins)' {
             $flag = [FeatureFlag]::new(@{