diff --git a/render.ts b/render.ts
index 653c1a8..05e9b20 100644
--- a/render.ts
+++ b/render.ts
@@ -125,7 +125,7 @@ function addReverseMapToken(
   state.manifest.tokens[token.result] = tokens
 }
 
-function getReverseMapPath(destRoot: string, reverseMap: boolean | string): string {
+function getReverseMapPath(destRoot: string, reverseMap: true | string): string {
   if (reverseMap === true) return resolveOutputPath(destRoot, ".tdir-map.json")
   return resolveOutputPath(destRoot, reverseMap)
 }