diff --git a/boring/src/x509/verify.rs b/boring/src/x509/verify.rs
index d07190f0..7546442d 100644
--- a/boring/src/x509/verify.rs
+++ b/boring/src/x509/verify.rs
@@ -112,7 +112,7 @@ impl X509VerifyParamRef {
 
     /// Gets verification flags.
     #[corresponds(X509_VERIFY_PARAM_get_flags)]
-    pub fn flags(&mut self) -> X509VerifyFlags {
+    pub fn flags(&self) -> X509VerifyFlags {
         let bits = unsafe { ffi::X509_VERIFY_PARAM_get_flags(self.as_ptr()) };
         X509VerifyFlags::from_bits_retain(bits)
     }