diff --git a/library/debug.c b/library/debug.c
index a9cd814be..f9229b360 100644
--- a/library/debug.c
+++ b/library/debug.c
@@ -71,7 +71,7 @@ static inline void debug_send_line( const mbedtls_ssl_context *ssl, int level,
      */
 #if defined(MBEDTLS_THREADING_C)
     char idstr[20 + DEBUG_BUF_SIZE]; /* 0x + 16 nibbles + ': ' */
-    mbedtls_snprintf( idstr, sizeof( idstr ), "%p: %s", ssl, str );
+    mbedtls_snprintf( idstr, sizeof( idstr ), "%p: %s", (void*)ssl, str );
     ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, idstr );
 #else
     ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );