diff --git a/CHANGELOG-v3.md b/CHANGELOG-v3.md
index 8c16f51b588..ba6238c3ec1 100644
--- a/CHANGELOG-v3.md
+++ b/CHANGELOG-v3.md
@@ -14,6 +14,7 @@
 - Fixed a bug where user email changes were going through email verification even if someone with permission to administrate users was making the change. ([#5088](https://github.com/craftcms/cms/issues/5088))
 - Fixed a bug where it wasn’t possible to eager-load Matrix blocks that belong to a draft or revision. ([#5031](https://github.com/craftcms/cms/issues/5031))
 - Fixed a bug where the `setup` command would think that Craft was installed when it wasn’t. ([#5093](https://github.com/craftcms/cms/issues/5093))
+- Fixed an error that could occur when syncing the project config, that could occur if a Matrix block had been changed to something else. ([#4015](https://github.com/craftcms/cms/issues/4015))
 
 ## 3.3.9 - 2019-10-10
 
diff --git a/src/services/Matrix.php b/src/services/Matrix.php
index f26d50d4da4..691cd4fea58 100644
--- a/src/services/Matrix.php
+++ b/src/services/Matrix.php
@@ -360,48 +360,53 @@ public function handleChangedBlockType(ConfigEvent $event)
             $contentService->fieldColumnPrefix = 'field_' . $blockTypeRecord->handle . '_';
             /** @var MatrixField $matrixField */
             $matrixField = $fieldsService->getFieldById($blockTypeRecord->fieldId);
-            $contentService->contentTable = $matrixField->contentTable;
-            $fieldsService->oldFieldColumnPrefix = 'field_' . ($blockTypeRecord->getOldAttribute('handle') ?? $data['handle']) . '_';
 
-            $oldFields = $previousData['fields'] ?? [];
-            $newFields = $data['fields'] ?? [];
+            // Ignore it, if the parent field is not a Matrix field.
+            if ($matrixField instanceof MatrixField) {
+                $contentService->contentTable = $matrixField->contentTable;
+                $fieldsService->oldFieldColumnPrefix = 'field_' . ($blockTypeRecord->getOldAttribute('handle') ?? $data['handle']) . '_';
+
+                $oldFields = $previousData['fields'] ?? [];
+                $newFields = $data['fields'] ?? [];
 
-            // Remove fields that this block type no longer has
-            foreach ($oldFields as $fieldUid => $fieldData) {
-                if (!array_key_exists($fieldUid, $newFields)) {
-                    $fieldsService->applyFieldDelete($fieldUid);
+                // Remove fields that this block type no longer has
+                foreach ($oldFields as $fieldUid => $fieldData) {
+                    if (!array_key_exists($fieldUid, $newFields)) {
+                        $fieldsService->applyFieldDelete($fieldUid);
+                    }
                 }
-            }
 
-            // (Re)save all the fields that now exist for this block.
-            foreach ($newFields as $fieldUid => $fieldData) {
-                $fieldsService->applyFieldSave($fieldUid, $fieldData, 'matrixBlockType:' . $blockTypeUid);
-            }
+                // (Re)save all the fields that now exist for this block.
+                foreach ($newFields as $fieldUid => $fieldData) {
+                    $fieldsService->applyFieldSave($fieldUid, $fieldData, 'matrixBlockType:' . $blockTypeUid);
+                }
 
-            // Refresh the schema cache
-            Craft::$app->getDb()->getSchema()->refresh();
+                // Refresh the schema cache
+                Craft::$app->getDb()->getSchema()->refresh();
+
+                $contentService->fieldContext = $originalFieldContext;
+                $contentService->fieldColumnPrefix = $originalFieldColumnPrefix;
+                $contentService->contentTable = $originalContentTable;
+                $fieldsService->oldFieldColumnPrefix = $originalOldFieldColumnPrefix;
+
+                if (!empty($data['fieldLayouts'])) {
+                    // Save the field layout
+                    $layout = FieldLayout::createFromConfig(reset($data['fieldLayouts']));
+                    $layout->id = $blockTypeRecord->fieldLayoutId;
+                    $layout->type = MatrixBlock::class;
+                    $layout->uid = key($data['fieldLayouts']);
+                    $fieldsService->saveLayout($layout);
+                    $blockTypeRecord->fieldLayoutId = $layout->id;
+                } else if ($blockTypeRecord->fieldLayoutId) {
+                    // Delete the field layout
+                    $fieldsService->deleteLayoutById($blockTypeRecord->fieldLayoutId);
+                    $blockTypeRecord->fieldLayoutId = null;
+                }
 
-            $contentService->fieldContext = $originalFieldContext;
-            $contentService->fieldColumnPrefix = $originalFieldColumnPrefix;
-            $contentService->contentTable = $originalContentTable;
-            $fieldsService->oldFieldColumnPrefix = $originalOldFieldColumnPrefix;
-
-            if (!empty($data['fieldLayouts'])) {
-                // Save the field layout
-                $layout = FieldLayout::createFromConfig(reset($data['fieldLayouts']));
-                $layout->id = $blockTypeRecord->fieldLayoutId;
-                $layout->type = MatrixBlock::class;
-                $layout->uid = key($data['fieldLayouts']);
-                $fieldsService->saveLayout($layout);
-                $blockTypeRecord->fieldLayoutId = $layout->id;
-            } else if ($blockTypeRecord->fieldLayoutId) {
-                // Delete the field layout
-                $fieldsService->deleteLayoutById($blockTypeRecord->fieldLayoutId);
-                $blockTypeRecord->fieldLayoutId = null;
+                // Save it
+                $blockTypeRecord->save(false);
             }
 
-            // Save it
-            $blockTypeRecord->save(false);
             $transaction->commit();
         } catch (\Throwable $e) {
             $transaction->rollBack();