diff --git a/examples/protocols/coap_client/main/coap_client_example_main.c b/examples/protocols/coap_client/main/coap_client_example_main.c
index c9d8b22fcd..98f003e1ad 100644
--- a/examples/protocols/coap_client/main/coap_client_example_main.c
+++ b/examples/protocols/coap_client/main/coap_client_example_main.c
@@ -139,7 +139,7 @@ static void coap_example_task(void *p)
                     FD_ZERO(&readfds);
                     FD_CLR( ctx->sockfd, &readfds );
                     FD_SET( ctx->sockfd, &readfds );
-                    result = select( FD_SETSIZE, &readfds, 0, 0, &tv );
+                    result = select( ctx->sockfd+1, &readfds, 0, 0, &tv );
                     if (result > 0) {
                         if (FD_ISSET( ctx->sockfd, &readfds ))
                             coap_read(ctx);
diff --git a/examples/protocols/coap_server/main/coap_server_example_main.c b/examples/protocols/coap_server/main/coap_server_example_main.c
index 47f47dfc5d..b3ea703080 100644
--- a/examples/protocols/coap_server/main/coap_server_example_main.c
+++ b/examples/protocols/coap_server/main/coap_server_example_main.c
@@ -120,7 +120,7 @@ static void coap_example_thread(void *p)
                     FD_CLR( ctx->sockfd, &readfds);
                     FD_SET( ctx->sockfd, &readfds);
 
-                    int result = select( FD_SETSIZE, &readfds, 0, 0, &tv );
+                    int result = select( ctx->sockfd+1, &readfds, 0, 0, &tv );
                     if (result > 0){
                         if (FD_ISSET( ctx->sockfd, &readfds ))
                             coap_read(ctx);