diff --git a/modules/Rulebook.Update.psm1 b/modules/Rulebook.Update.psm1
index 218467d..fe0e7a7 100644
--- a/modules/Rulebook.Update.psm1
+++ b/modules/Rulebook.Update.psm1
@@ -1002,6 +1002,9 @@ function Get-EffectiveDiffBlock {
     .DESCRIPTION
     The rendering of Validate.ps1, shared by the update and the scan pull request bodies. Endpoints in the order of
     -Diff (settings order).
+    .NOTES
+    The array is returned with a leading comma so that a one-element result stays an array. Callers assign it
+    directly or to [string[]] and never wrap the call in @(), which would nest it and print System.String[] (#75).
     #>
     [CmdletBinding()]
     [OutputType([string[]])]
@@ -1226,7 +1229,9 @@ function ConvertTo-UpdateSummary {
         [void]$text.AppendLine('## Effective diff').AppendLine().AppendLine($diffNote).AppendLine()
     } elseif ($null -ne $Result -and @($Result.Diff).Count -gt 0) {
         [void]$text.AppendLine('## Effective diff').AppendLine()
-        [void]$text.Append((@(Get-EffectiveDiffBlock -Diff @($Result.Diff)) -join ''))
+        # Get-EffectiveDiffBlock returns its array with a comma; @() around it would nest it and print System.String[] (#75).
+        [string[]]$blocks = Get-EffectiveDiffBlock -Diff @($Result.Diff)
+        [void]$text.Append(($blocks -join ''))
     }
     if ($Plan.ReleaseNotesShipped -and -not [string]::IsNullOrWhiteSpace($Plan.ReleaseNotes)) {
         [void]$text.AppendLine('## Release notes').AppendLine().AppendLine((ConvertTo-ReleaseNotesMarkdown -Text $Plan.ReleaseNotes)).AppendLine()
diff --git a/tests/Rulebook.Update.Tests.ps1 b/tests/Rulebook.Update.Tests.ps1
index a7594d4..c6b207f 100644
--- a/tests/Rulebook.Update.Tests.ps1
+++ b/tests/Rulebook.Update.Tests.ps1
@@ -1086,6 +1086,14 @@ Describe 'ConvertTo-UpdatePullRequestBody and ConvertTo-UpdateSummary' {
         $summary | Should-MatchString '(?m)^## Release notes\n\n### v1\.1$'
     }
 
+    It 'writes the effective diff tables into the summary, not System.String[] (#75)' {
+        $row = [pscustomobject]@{ Endpoint = 'recommended.default'; File = 'rulesets/recommended.ruleset.json'; Id = 'LC0031'; Before = 'Warning'; After = 'Error'; BeforeSource = 'default'; AfterSource = 'level:recommended'; AfterDetail = $null; Change = 'action'; ListedAfter = $true }
+        $result = [pscustomobject]@{ Result = 'pull-request'; PullRequestUrl = 'https://github.com/Contoso/rulebook/pull/3'; Fallback = $false; Diff = @($row) }
+        $summary = ConvertTo-UpdateSummary -Plan $plan -Result $result -Mode update
+        $summary | Should-MatchString '(?m)^## Effective diff\n\n### `recommended\.default` \(`rulesets/recommended\.ruleset\.json`\)\n\n\| Id \| Before \| After \| Decided by \|\n\|---\|---\|---\|---\|\n\| LC0031 \| Warning \| Error \| level:recommended \|$'
+        $summary | Should-NotMatchString 'System\.String\[\]'
+    }
+
     It 'writes the check summary with the change table' {
         $summary = ConvertTo-UpdateSummary -Plan $plan -Mode check -Message 'Updates available'
         $summary | Should-MatchString '(?m)^## Template update check$'