diff --git a/TAP.idr b/TAP.idr
index c70a544..fdc3459 100644
--- a/TAP.idr
+++ b/TAP.idr
@@ -189,7 +189,8 @@ header tests =
 private
 withPrefix : SnocList String -> TestBase -> Test
 withPrefix pfx = One . {label $= (makePrefix pfx ++)}
-  where makePrefix = concatMap $ \s => "\{s} » "
+  where makePrefix : SnocList String -> String
+        makePrefix = concatMap $ \s => "\{s} » "
 
 mutual
   ||| flatten some tests, starting with the prefix given