diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index cb5a697..1ffb876 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -62,7 +62,7 @@
  * DES       1  0x0032-0x0032
  * CTR_DBRG  4  0x0034-0x003A
  * ENTROPY   3  0x003C-0x0040   0x003D-0x003F
- * NET       9  0x0042-0x0052   0x0043-0x0043
+ * NET      11  0x0042-0x0052   0x0043-0x0045
  * ASN1      7  0x0060-0x006C
  * PBKDF2    1  0x007C-0x007C
  * HMAC_DRBG 4  0x0003-0x0009
diff --git a/include/mbedtls/net.h b/include/mbedtls/net.h
index 19746e5..1c49763 100644
--- a/include/mbedtls/net.h
+++ b/include/mbedtls/net.h
@@ -45,6 +45,7 @@
 #define MBEDTLS_ERR_NET_CONN_RESET                        -0x0050  /**< Connection was reset by peer. */
 #define MBEDTLS_ERR_NET_UNKNOWN_HOST                      -0x0052  /**< Failed to get an IP address for the given hostname. */
 #define MBEDTLS_ERR_NET_BUFFER_TOO_SMALL                  -0x0043  /**< Buffer is too small to hold the data. */
+#define MBEDTLS_ERR_NET_INVALID_CONTEXT                   -0x0045  /**< The context is invalid, eg because it was free()ed. */
 
 #define MBEDTLS_NET_LISTEN_BACKLOG         10 /**< The backlog that listen() should use. */
 
diff --git a/library/error.c b/library/error.c
index 21be423..e78057b 100644
--- a/library/error.c
+++ b/library/error.c
@@ -640,6 +640,8 @@
         mbedtls_snprintf( buf, buflen, "NET - Failed to get an IP address for the given hostname" );
     if( use_ret == -(MBEDTLS_ERR_NET_BUFFER_TOO_SMALL) )
         mbedtls_snprintf( buf, buflen, "NET - Buffer is too small to hold the data" );
+    if( use_ret == -(MBEDTLS_ERR_NET_INVALID_CONTEXT) )
+        mbedtls_snprintf( buf, buflen, "NET - The context is invalid, eg because it was free()ed" );
 #endif /* MBEDTLS_NET_C */
 
 #if defined(MBEDTLS_OID_C)
diff --git a/library/net.c b/library/net.c
index 3c69646..bcec232 100644
--- a/library/net.c
+++ b/library/net.c
@@ -448,8 +448,13 @@
  */
 int mbedtls_net_recv( void *ctx, unsigned char *buf, size_t len )
 {
+    int ret;
     int fd = ((mbedtls_net_context *) ctx)->fd;
-    int ret = (int) read( fd, buf, len );
+
+    if( fd < 0 )
+        return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
+
+    ret = (int) read( fd, buf, len );
 
     if( ret < 0 )
     {
@@ -485,6 +490,9 @@
     fd_set read_fds;
     int fd = ((mbedtls_net_context *) ctx)->fd;
 
+    if( fd < 0 )
+        return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
+
     FD_ZERO( &read_fds );
     FD_SET( fd, &read_fds );
 
@@ -520,8 +528,13 @@
  */
 int mbedtls_net_send( void *ctx, const unsigned char *buf, size_t len )
 {
+    int ret;
     int fd = ((mbedtls_net_context *) ctx)->fd;
-    int ret = (int) write( fd, buf, len );
+
+    if( fd < 0 )
+        return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
+
+    ret = (int) write( fd, buf, len );
 
     if( ret < 0 )
     {