diff --git a/openssl/src/ssl/mod.rs b/openssl/src/ssl/mod.rs
index b4c73479d59e2e70c72b2e159daa141ddd0ea0f9..574a324b0d3488937c1d8cfdfd22733713ea1343 100644
--- a/openssl/src/ssl/mod.rs
+++ b/openssl/src/ssl/mod.rs
@@ -569,7 +569,8 @@ impl SslContext {
     pub fn set_servername_callback(&mut self, callback: Option<ServerNameCallback>) {
         unsafe {
             ffi::SSL_CTX_set_ex_data(self.ctx, SNI_IDX, mem::transmute(callback));
-            let f: extern "C" fn() = mem::transmute(raw_sni);
+            let f: extern "C" fn(_, _, _) -> _ = raw_sni;
+            let f: extern "C" fn() = mem::transmute(f);
             ffi_extras::SSL_CTX_set_tlsext_servername_callback(self.ctx, Some(f));
         }
     }
@@ -586,7 +587,8 @@ impl SslContext {
             ffi::SSL_CTX_set_ex_data(self.ctx, SNI_IDX, mem::transmute(Some(callback)));
 
             ffi_extras::SSL_CTX_set_tlsext_servername_arg(self.ctx, mem::transmute(data));
-            let f: extern "C" fn() = mem::transmute(raw_sni_with_data::<T>);
+            let f: extern "C" fn(_, _, _) -> _ = raw_sni_with_data::<T>;
+            let f: extern "C" fn() = mem::transmute(f);
             ffi_extras::SSL_CTX_set_tlsext_servername_callback(self.ctx, Some(f));
         }
     }