<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**: @aaandre 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 Brad Bell 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  

---

