diff --git a/src/resources/extensions/gsd/milestone-ids.ts b/src/resources/extensions/gsd/milestone-ids.ts
index aec8cee67..fdd26f7ab 100644
--- a/src/resources/extensions/gsd/milestone-ids.ts
+++ b/src/resources/extensions/gsd/milestone-ids.ts
@@ -80,8 +80,9 @@ export function findMilestoneIds(basePath: string): string[] {
       .filter((d) => d.isDirectory())
       .map((d) => {
         const match = d.name.match(/^(M\d+(?:-[a-z0-9]{6})?)/);
-        return match ? match[1] : d.name;
-      });
+        return match ? match[1] : null;
+      })
+      .filter((id): id is string => id !== null);
 
     // Apply custom queue order if available, else fall back to numeric sort
     const customOrder = loadQueueOrder(basePath);
diff --git a/src/resources/extensions/gsd/tests/queue-reorder-e2e.test.ts b/src/resources/extensions/gsd/tests/queue-reorder-e2e.test.ts
index b9140c561..bf86c360a 100644
--- a/src/resources/extensions/gsd/tests/queue-reorder-e2e.test.ts
+++ b/src/resources/extensions/gsd/tests/queue-reorder-e2e.test.ts
@@ -244,6 +244,32 @@ console.log('\n=== E2E: backward compat without QUEUE-ORDER.json ===');
   }
 }
 
+// ═══════════════════════════════════════════════════════════════════════════
+// Test: non-milestone directories are filtered out (#1494)
+// ═══════════════════════════════════════════════════════════════════════════
+
+console.log('\n=== E2E: non-milestone directories filtered from findMilestoneIds (#1494) ===');
+{
+  const base = createFixtureBase();
+  try {
+    writeContext(base, 'M001', '', 'First');
+    writeContext(base, 'M002', '', 'Second');
+    // Create a rogue non-milestone directory
+    mkdirSync(join(base, '.gsd', 'milestones', 'slices'), { recursive: true });
+    mkdirSync(join(base, '.gsd', 'milestones', 'temp-backup'), { recursive: true });
+
+    invalidateStateCache();
+    const ids = findMilestoneIds(base);
+    assertEq(ids.length, 2, 'only M001 and M002 returned');
+    assertTrue(!ids.includes('slices'), 'slices directory excluded');
+    assertTrue(!ids.includes('temp-backup'), 'temp-backup directory excluded');
+    assertTrue(ids.includes('M001'), 'M001 included');
+    assertTrue(ids.includes('M002'), 'M002 included');
+  } finally {
+    cleanup(base);
+  }
+}
+
 // ═══════════════════════════════════════════════════════════════════════════
 // Test: depends_on inline array format removal
 // ═══════════════════════════════════════════════════════════════════════════