diff --git a/CHANGELOG.md b/CHANGELOG.md
index 60cb12922a7..bc2eed5a487 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Release Notes for Craft CMS 5
 
+## Unreleased
+
+- Fixed a bug where asset, category, and entry sources defined by the `EVENT_REGISTER_SOURCES` event didn’t have any custom fields available to them, unless the `EVENT_REGISTER_FIELD_LAYOUTS` event was also used to define the available field layouts for the event-defined source. ([#16256](https://github.com/craftcms/cms/discussions/16256))
+
 ## 5.5.4 - 2024-12-02
 
 - Reduced the likelihood of a deadlock error occurring when updating search indexes. ([#15221](https://github.com/craftcms/cms/issues/15221))
diff --git a/src/elements/Asset.php b/src/elements/Asset.php
index 90ff6024c7f..f45b35702bf 100644
--- a/src/elements/Asset.php
+++ b/src/elements/Asset.php
@@ -447,14 +447,10 @@ public static function sourcePath(string $sourceKey, string $stepKey, ?string $c
      */
     protected static function defineFieldLayouts(?string $source): array
     {
-        if ($source !== null) {
-            $volumes = [];
-            if (preg_match('/^volume:(.+)$/', $source, $matches)) {
-                $volume = Craft::$app->getVolumes()->getVolumeByUid($matches[1]);
-                if ($volume) {
-                    $volumes[] = $volume;
-                }
-            }
+        if ($source !== null && preg_match('/^volume:(.+)$/', $source, $matches)) {
+            $volumes = array_filter([
+                Craft::$app->getVolumes()->getVolumeByUid($matches[1]),
+            ]);
         } else {
             $volumes = Craft::$app->getVolumes()->getAllVolumes();
         }
diff --git a/src/elements/Category.php b/src/elements/Category.php
index cff3c51739c..a1276195f2f 100644
--- a/src/elements/Category.php
+++ b/src/elements/Category.php
@@ -200,14 +200,10 @@ protected static function defineSources(string $context): array
      */
     protected static function defineFieldLayouts(?string $source): array
     {
-        if ($source !== null) {
-            $groups = [];
-            if (preg_match('/^group:(.+)$/', $source, $matches)) {
-                $group = Craft::$app->getCategories()->getGroupByUid($matches[1]);
-                if ($group) {
-                    $groups[] = $group;
-                }
-            }
+        if ($source !== null && preg_match('/^group:(.+)$/', $source, $matches)) {
+            $groups = array_filter([
+                Craft::$app->getCategories()->getGroupByUid($matches[1]),
+            ]);
         } else {
             $groups = Craft::$app->getCategories()->getAllGroups();
         }
diff --git a/src/elements/Entry.php b/src/elements/Entry.php
index b04cbad5fa2..63936735824 100644
--- a/src/elements/Entry.php
+++ b/src/elements/Entry.php
@@ -371,21 +371,17 @@ public static function modifyCustomSource(array $config): array
      */
     protected static function defineFieldLayouts(?string $source): array
     {
-        if ($source !== null) {
-            if ($source === '*') {
-                $sections = Craft::$app->getEntries()->getAllSections();
-            } elseif ($source === 'singles') {
-                $sections = Craft::$app->getEntries()->getSectionsByType(Section::TYPE_SINGLE);
-            } else {
-                $sections = [];
-                if (preg_match('/^section:(.+)$/', $source, $matches)) {
-                    $section = Craft::$app->getEntries()->getSectionByUid($matches[1]);
-                    if ($section) {
-                        $sections[] = $section;
-                    }
-                }
-            }
+        if ($source === '*') {
+            $sections = Craft::$app->getEntries()->getAllSections();
+        } elseif ($source === 'singles') {
+            $sections = Craft::$app->getEntries()->getSectionsByType(Section::TYPE_SINGLE);
+        } elseif ($source !== null && preg_match('/^section:(.+)$/', $source, $matches)) {
+            $sections = array_filter([
+                Craft::$app->getEntries()->getSectionByUid($matches[1]),
+            ]);
+        }
 
+        if (isset($sections)) {
             $entryTypes = array_values(array_unique(array_merge(
                 ...array_map(fn(Section $section) => $section->getEntryTypes(), $sections),
             )));