diff --git a/src/coap_io.c b/src/coap_io.c
index 9af6d98e..f435f47b 100644
--- a/src/coap_io.c
+++ b/src/coap_io.c
@@ -1169,7 +1169,7 @@ coap_run_once(coap_context_t *ctx, unsigned timeout_ms) {
 #else
     if (errno != EINTR) {
 #endif
-      coap_log(LOG_DEBUG, coap_socket_strerror());
+      coap_log(LOG_DEBUG, "%s", coap_socket_strerror());
       return -1;
     }
   }