diff --git a/src/headerdef.rs b/src/headerdef.rs
index 30d01222b..6f9dd495a 100644
--- a/src/headerdef.rs
+++ b/src/headerdef.rs
@@ -70,7 +70,8 @@ pub enum HeaderDef {
     /// See <https://datatracker.ietf.org/doc/html/rfc8601>
     AuthenticationResults,
 
-    _TestHeader,
+    #[cfg(test)]
+    TestHeader,
 }
 
 impl HeaderDef {
@@ -103,7 +104,7 @@ mod tests {
     fn kebab_test() {
         assert_eq!(HeaderDef::From_.get_headername(), "from");
 
-        assert_eq!(HeaderDef::_TestHeader.get_headername(), "test-header");
+        assert_eq!(HeaderDef::TestHeader.get_headername(), "test-header");
     }
 
     #[test]
diff --git a/src/mimeparser.rs b/src/mimeparser.rs
index d894ebc4d..92c43221d 100644
--- a/src/mimeparser.rs
+++ b/src/mimeparser.rs
@@ -2362,7 +2362,7 @@ mod tests {
         assert_eq!(of, "no");
 
         // unknown headers do not bubble upwards
-        let of = mimeparser.get_header(HeaderDef::_TestHeader).unwrap();
+        let of = mimeparser.get_header(HeaderDef::TestHeader).unwrap();
         assert_eq!(of, "Bar");
 
         // the following fields would bubble up