diff --git a/src/layers/perimeters/source.js b/src/layers/perimeters/source.js
index b654fc651..569191781 100644
--- a/src/layers/perimeters/source.js
+++ b/src/layers/perimeters/source.js
@@ -8,36 +8,35 @@ const msg = (text) => text;
 export function createWfigsPerimeterSource({
   fetchImpl = (...args) => globalThis.fetch(...args),
 } = {}) {
+  /**
+   * The rows with how fresh they are: the proxy answers from its last
+   * snapshot, marked stale, while WFIGS is unreachable.
+   * @returns {Promise<{rows: object[], stale: boolean, fetchedAt: number|null}>}
+   */
+  async function getSnapshotDetail({ signal } = {}) {
+    signal?.throwIfAborted();
+    const response = await fetchImpl('/api/fire-perimeters', { signal });
+    if (!response.ok) throw new Error(`WFIGS HTTP ${response.status}`);
+    const payload = await readResponseJsonCapped(
+      response,
+      80 * 1024 * 1024,
+      signal,
+    );
+    signal?.throwIfAborted();
+    if (!Array.isArray(payload?.rows))
+      throw new Error(msg('Malformed perimeter snapshot'));
+    return {
+      rows: payload.rows,
+      stale: payload.stale === true,
+      fetchedAt: Number.isFinite(payload.fetchedAt) ? payload.fetchedAt : null,
+    };
+  }
+
   return {
     /** The perimeter rows (see getSnapshotDetail for staleness). */
     async getSnapshot(options = {}) {
-      return (await this.getSnapshotDetail(options)).rows;
-    },
-
-    /**
-     * The rows with how fresh they are: the proxy answers from its last
-     * snapshot, marked stale, while WFIGS is unreachable.
-     * @returns {Promise<{rows: object[], stale: boolean, fetchedAt: number|null}>}
-     */
-    async getSnapshotDetail({ signal } = {}) {
-      signal?.throwIfAborted();
-      const response = await fetchImpl('/api/fire-perimeters', { signal });
-      if (!response.ok) throw new Error(`WFIGS HTTP ${response.status}`);
-      const payload = await readResponseJsonCapped(
-        response,
-        80 * 1024 * 1024,
-        signal,
-      );
-      signal?.throwIfAborted();
-      if (!Array.isArray(payload?.rows))
-        throw new Error(msg('Malformed perimeter snapshot'));
-      return {
-        rows: payload.rows,
-        stale: payload.stale === true,
-        fetchedAt: Number.isFinite(payload.fetchedAt)
-          ? payload.fetchedAt
-          : null,
-      };
+      return (await getSnapshotDetail(options)).rows;
     },
+    getSnapshotDetail,
   };
 }
diff --git a/src/layers/perimeters/source.test.mjs b/src/layers/perimeters/source.test.mjs
index 3944fe77b..bb6d6b305 100644
--- a/src/layers/perimeters/source.test.mjs
+++ b/src/layers/perimeters/source.test.mjs
@@ -172,3 +172,10 @@ test('the snapshot detail carries the proxy staleness and fetch time', async ()
   });
   assert.deepEqual(await source.getSnapshot(), []);
 });
+
+test('getSnapshot works when taken off its source object', async () => {
+  const { getSnapshot } = createWfigsPerimeterSource({
+    fetchImpl: async () => Response.json({ rows: [] }),
+  });
+  assert.deepEqual(await getSnapshot(), []);
+});