diff --git a/src/Fieldtypes/SourceFieldtype.php b/src/Fieldtypes/SourceFieldtype.php
index a6c9cb80..db4e7ed8 100755
--- a/src/Fieldtypes/SourceFieldtype.php
+++ b/src/Fieldtypes/SourceFieldtype.php
@@ -36,6 +36,13 @@ public function preProcess($data)
             return ['source' => 'inherit', 'value' => $data];
         }
 
+        // Handle issue with legacy `sitemap: true` section default.
+        // This shouldn't ever be explicitly set `true` in Statamic v3,
+        // but it may be migrated as `true` when coming from Statamic v2.
+        if ($this->field->handle() === 'sitemap' && $originalData === true) {
+            return ['source' => 'inherit', 'value' => null];
+        }
+
         return ['source' => 'custom', 'value' => $data];
     }