diff --git a/lib/imap.c b/lib/imap.c
index 76b78a12a8..7027c8ef05 100644
--- a/lib/imap.c
+++ b/lib/imap.c
@@ -154,6 +154,9 @@ struct ulbits {
  *
  * Designed to never block.
  */
+static CURLcode imap_sendf(struct Curl_easy *data,
+                           struct imap_conn *imapc,
+                           const char *fmt, ...) CURL_PRINTF(3, 0);
 static CURLcode imap_sendf(struct Curl_easy *data,
                            struct imap_conn *imapc,
                            const char *fmt, ...)