diff --git a/src/SingleMode.hxx b/src/SingleMode.hxx
index 91a9a2619..e9563d961 100644
--- a/src/SingleMode.hxx
+++ b/src/SingleMode.hxx
@@ -31,7 +31,7 @@ enum class SingleMode : uint8_t {
 /**
  * Return the string representation of a #SingleMode.
  */
-[[gnu::pure]]
+[[gnu::const]]
 const char *
 SingleToString(SingleMode mode) noexcept;