diff --git a/src/api/err.c b/src/api/err.c
index 6701c61e..b0a4eb3e 100644
--- a/src/api/err.c
+++ b/src/api/err.c
@@ -48,14 +48,15 @@ static const char *err_strerr[] = {
            "Routing problem.",       /* ERR_RTE        -4  */
            "Operation in progress.", /* ERR_INPROGRESS -5  */
            "Illegal value.",         /* ERR_VAL        -6  */
-           "Connection aborted.",    /* ERR_ABRT       -7  */
-           "Connection reset.",      /* ERR_RST        -8  */
-           "Connection closed.",     /* ERR_CLSD       -9  */
-           "Not connected.",         /* ERR_CONN       -10 */
-           "Illegal argument.",      /* ERR_ARG        -11 */
-           "Address in use.",        /* ERR_USE        -12 */
-           "Low-level netif error.", /* ERR_IF         -13 */
-           "Already connected.",     /* ERR_ISCONN     -14 */
+           "Operation would block.", /* ERR_WOULDBLOCK -7  */
+           "Connection aborted.",    /* ERR_ABRT       -8  */
+           "Connection reset.",      /* ERR_RST        -9  */
+           "Connection closed.",     /* ERR_CLSD       -10 */
+           "Not connected.",         /* ERR_CONN       -11 */
+           "Illegal argument.",      /* ERR_ARG        -12 */
+           "Address in use.",        /* ERR_USE        -13 */
+           "Low-level netif error.", /* ERR_IF         -14 */
+           "Already connected.",     /* ERR_ISCONN     -15 */
 };
 
 /**
diff --git a/src/api/sockets.c b/src/api/sockets.c
index 1162ece8..12591e5a 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -130,14 +130,15 @@ static const int err_to_errno_table[] = {
   EHOSTUNREACH,  /* ERR_RTE        -4      Routing problem.         */
   EINPROGRESS,   /* ERR_INPROGRESS -5      Operation in progress    */
   EINVAL,        /* ERR_VAL        -6      Illegal value.           */
-  ECONNABORTED,  /* ERR_ABRT       -7      Connection aborted.      */
-  ECONNRESET,    /* ERR_RST        -8      Connection reset.        */
-  ESHUTDOWN,     /* ERR_CLSD       -9      Connection closed.       */
-  ENOTCONN,      /* ERR_CONN       -10     Not connected.           */
-  EIO,           /* ERR_ARG        -11     Illegal argument.        */
-  EADDRINUSE,    /* ERR_USE        -12     Address in use.          */
-  -1,            /* ERR_IF         -13     Low-level netif error    */
-  -1,            /* ERR_ISCONN     -14     Already connected.       */
+  EWOULDBLOCK,   /* ERR_WOULBLOCK  -7      Operation would block.   */
+  ECONNABORTED,  /* ERR_ABRT       -8      Connection aborted.      */
+  ECONNRESET,    /* ERR_RST        -9      Connection reset.        */
+  ESHUTDOWN,     /* ERR_CLSD       -10     Connection closed.       */
+  ENOTCONN,      /* ERR_CONN       -11     Not connected.           */
+  EIO,           /* ERR_ARG        -12     Illegal argument.        */
+  EADDRINUSE,    /* ERR_USE        -13     Address in use.          */
+  -1,            /* ERR_IF         -14     Low-level netif error    */
+  -1,            /* ERR_ISCONN     -15     Already connected.       */
 };
 
 #define ERR_TO_ERRNO_TABLE_SIZE \
diff --git a/src/include/lwip/err.h b/src/include/lwip/err.h
index 91916a71..b4fe0502 100644
--- a/src/include/lwip/err.h
+++ b/src/include/lwip/err.h
@@ -56,20 +56,21 @@ typedef s8_t err_t;
 #define ERR_RTE        -4    /* Routing problem.         */
 #define ERR_INPROGRESS -5    /* Operation in progress    */
 #define ERR_VAL        -6    /* Illegal value.           */
+#define ERR_WOULBLOCK  -7    /* Operation would block.   */
 
 #define ERR_IS_FATAL(e) ((e) < ERR_VAL)
 
-#define ERR_ABRT       -7    /* Connection aborted.      */
-#define ERR_RST        -8    /* Connection reset.        */
-#define ERR_CLSD       -9    /* Connection closed.       */
-#define ERR_CONN       -10   /* Not connected.           */
+#define ERR_ABRT       -8    /* Connection aborted.      */
+#define ERR_RST        -9    /* Connection reset.        */
+#define ERR_CLSD       -10   /* Connection closed.       */
+#define ERR_CONN       -11   /* Not connected.           */
 
-#define ERR_ARG        -11   /* Illegal argument.        */
+#define ERR_ARG        -12   /* Illegal argument.        */
 
-#define ERR_USE        -12   /* Address in use.          */
+#define ERR_USE        -13   /* Address in use.          */
 
-#define ERR_IF         -13   /* Low-level netif error    */
-#define ERR_ISCONN     -14   /* Already connected.       */
+#define ERR_IF         -14   /* Low-level netif error    */
+#define ERR_ISCONN     -15   /* Already connected.       */
 
 
 #ifdef LWIP_DEBUG