diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 1a3102a..0460357 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -313,13 +313,13 @@
  * \param end    Pointer to one past the end of the buffer.
  * \param need   Needed space in bytes.
  *
- * \return       Non-zero if the needed space is available in the buffer, 0
+ * \return       Zero if the needed space is available in the buffer, non-zero
  *               otherwise.
  */
 static inline int mbedtls_ssl_chk_buf_ptr( const uint8_t *cur,
                                            const uint8_t *end, size_t need )
 {
-    return( cur <= end && need <= (size_t)( end - cur ) );
+    return( ( cur > end ) || ( need > (size_t)( end - cur ) ) );
 }
 
 /**
@@ -334,7 +334,7 @@
  */
 #define MBEDTLS_SSL_CHK_BUF_PTR( cur, end, need )                        \
     do {                                                                 \
-        if( mbedtls_ssl_chk_buf_ptr( ( cur ), ( end ), ( need ) ) == 0 ) \
+        if( mbedtls_ssl_chk_buf_ptr( ( cur ), ( end ), ( need ) ) != 0 ) \
         {                                                                \
             return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );                  \
         }                                                                \