<p>This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by Brad Bell in his comment on the original question.</p>

<p>Upgrading to Craft 3.3.16.1 fixes this issue.</p>

<p>(I'm posting this as an answer in case anybody else runs into the same issue.)</p>

CC BY-SA 4.0
2019-12-01T20:42:23.080Z
The upgrade fixed the issue for me, too
aaandre
CC BY-SA 4.0
2019-11-23T10:48:56.703Z
2019-11-23T10:48:56.703Z
Mark Northrop
<p>Craft 3.3.15</p>

<p>Previous version of the entry was saved 1 minute ago, so no changes in settings between the two entries.</p>

<p>Any ideas on how to debug this?</p>

<p>Update - here's the error, which looks very much like what @mtnorthrop is getting:</p>

<pre><code>2019-11-20 10:41:44 [-][1][-][error][TypeError] TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/helpers/ElementHelper.php:47
</code></pre>

CC BY-SA 4.0
2019-11-15T05:14:28.330Z
Check your Craft `storage/logs` files for `[error]` - that should contain the underlying error message for the "unknown error" flash message.
Brad Bell
CC BY-SA 4.0
2019-11-21T10:25:02.163Z
I've also experienced this issue. The logs report: TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /vendor/craftcms/cms/src/helpers/ElementHelper.php:47
Mark Northrop
CC BY-SA 4.0
2019-11-21T22:18:19.530Z
@mtnorthrop did you find a solution?
aaandre
CC BY-SA 4.0
2019-11-23T00:26:39.157Z
This was a bug in Craft that's been fixed as of Craft 3.3.16: https://github.com/craftcms/cms/blob/develop/CHANGELOG-v3.md#3316---2019-11-22 Going to go ahead and vote to close this as a bug report.
Brad Bell
CC BY-SA 4.0
2019-11-23T10:42:15.437Z
@aaandre I can confirm that updating to Craft 3.3.16.1 fixed this issue for me.
Mark Northrop
<error><bug><entry-versions><preview>
If I try to preview a previous revision of an entry, I get "an unknown error occurred"
# If I try to preview a previous revision of an entry, I get "an unknown error occurred"

(Asked by: username_0 on 2019-11-13)

Craft 3.3.15
Previous version of the entry was saved 1 minute ago, so no changes in settings between the two entries.
Any ideas on how to debug this?
Update - here's the error, which looks very much like what @mtnorthrop is getting:
2019-11-20 10:41:44 [-][1][-][error][TypeError] TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/helpers/ElementHelper.php:47

**Tags:** error, bug, entry-versions, preview

> **Comments:**
> **username_1**: Check your Craft `storage/logs` files for `[error]` - that should contain the underlying error message for the "unknown error" flash message.  
> **username_2**: I've also experienced this issue. The logs report: TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /vendor/craftcms/cms/src/helpers/ElementHelper.php:47  
> **username_0**: @mtnorthrop did you find a solution?  
> **username_1**: This was a bug in Craft that's been fixed as of Craft 3.3.16: https://github.com/craftcms/cms/blob/develop/CHANGELOG-v3.md#3316---2019-11-22 Going to go ahead and vote to close this as a bug report.  
> **username_2**: @username_0 I can confirm that updating to Craft 3.3.16.1 fixed this issue for me.  

---

## 1 answers

### Answer by username_2 on 2019-11-23. Score: 2 | Accepted answer

This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by username_1 in his comment on the original question.

Upgrading to Craft 3.3.16.1 fixes this issue.

(I'm posting this as an answer in case anybody else runs into the same issue.)

> **Comments:**
> **username_0**: The upgrade fixed the issue for me, too  

---

