diff --git a/CHANGELOG.md b/CHANGELOG.md
index 504590d55b..5899e1e894 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -161,6 +161,8 @@ The file documents changes to the PHP_CodeSniffer project.
     - Thanks to Juliette Reinders Folmer (@jrfnl) for the patch
 - Fixed bug #3906 : Tokenizer/CSS: fixed a bug related to the unsupported slash comment syntax
     - Thanks to Dan Wallis (@fredden) for the patch
+- Fixed bug #3913 : Config stored unknown "long" arguments in a (dynamic) `$values` property
+    - Thanks to Juliette Reinders Folmer (@jrfnl) for the patch
 
 Props also to Dan Wallis (@fredden) and Danny van der Sluijs (@DannyvdSluijs) for reviewing quite a few of the PRs for this release.
 
diff --git a/src/Config.php b/src/Config.php
index 55c7598f2b..ee50bc79d9 100644
--- a/src/Config.php
+++ b/src/Config.php
@@ -1232,13 +1232,17 @@ public function processLongArgument($arg, $pos)
                 if ($this->dieOnUnknownArg === false) {
                     $eqPos = strpos($arg, '=');
                     try {
+                        $unknown = $this->unknown;
+
                         if ($eqPos === false) {
-                            $this->values[$arg] = $arg;
+                            $unknown[$arg] = $arg;
                         } else {
-                            $value = substr($arg, ($eqPos + 1));
-                            $arg   = substr($arg, 0, $eqPos);
-                            $this->values[$arg] = $value;
+                            $value         = substr($arg, ($eqPos + 1));
+                            $arg           = substr($arg, 0, $eqPos);
+                            $unknown[$arg] = $value;
                         }
+
+                        $this->unknown = $unknown;
                     } catch (RuntimeException $e) {
                         // Value is not valid, so just ignore it.
                     }