diff --git a/src/impl_phf.rs b/src/impl_phf.rs
index 5f44818..f35fb94 100644
--- a/src/impl_phf.rs
+++ b/src/impl_phf.rs
@@ -15,6 +15,7 @@ pub fn get_mime_types(ext: &str) -> Option<&'static [&'static str]> {
     map_lookup(&MIME_TYPES, ext).cloned()
 }
 
+#[cfg(feature = "rev-map")]
 pub fn get_extensions(toplevel: &str, sublevel: &str) -> Option<&'static [&'static str]> {
     if toplevel == "*" {
         return Some(EXTS);