diff --git a/ChangeLog b/ChangeLog
index 07933fd35..7c23ffc9f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -55,6 +55,8 @@ Bugfix
      in the internal buffers; these cases lead to deadlocks in case
      event-driven I/O was used.
      Found and reported by Hubert Mis in #772.
+   * Fix invalid buffer sizes passed to zlib during record compression and
+     decompression.
 
 Changes
    * Remove some redundant code in bignum.c. Contributed by Alexey Skalozub.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index e8063d2c1..8a903c563 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2108,6 +2108,7 @@ static int ssl_compress_buf( mbedtls_ssl_context *ssl )
 {
     int ret;
     unsigned char *msg_post = ssl->out_msg;
+    ptrdiff_t bytes_written = ssl->out_msg - ssl->out_buf;
     size_t len_pre = ssl->out_msglen;
     unsigned char *msg_pre = ssl->compress_buf;
 
@@ -2127,7 +2128,7 @@ static int ssl_compress_buf( mbedtls_ssl_context *ssl )
     ssl->transform_out->ctx_deflate.next_in = msg_pre;
     ssl->transform_out->ctx_deflate.avail_in = len_pre;
     ssl->transform_out->ctx_deflate.next_out = msg_post;
-    ssl->transform_out->ctx_deflate.avail_out = MBEDTLS_SSL_BUFFER_LEN;
+    ssl->transform_out->ctx_deflate.avail_out = MBEDTLS_SSL_BUFFER_LEN - bytes_written;
 
     ret = deflate( &ssl->transform_out->ctx_deflate, Z_SYNC_FLUSH );
     if( ret != Z_OK )
@@ -2137,7 +2138,7 @@ static int ssl_compress_buf( mbedtls_ssl_context *ssl )
     }
 
     ssl->out_msglen = MBEDTLS_SSL_BUFFER_LEN -
-                      ssl->transform_out->ctx_deflate.avail_out;
+                      ssl->transform_out->ctx_deflate.avail_out - bytes_written;
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "after compression: msglen = %d, ",
                    ssl->out_msglen ) );
@@ -2154,6 +2155,7 @@ static int ssl_decompress_buf( mbedtls_ssl_context *ssl )
 {
     int ret;
     unsigned char *msg_post = ssl->in_msg;
+    ptrdiff_t header_bytes = ssl->in_msg - ssl->in_buf;
     size_t len_pre = ssl->in_msglen;
     unsigned char *msg_pre = ssl->compress_buf;
 
@@ -2173,7 +2175,8 @@ static int ssl_decompress_buf( mbedtls_ssl_context *ssl )
     ssl->transform_in->ctx_inflate.next_in = msg_pre;
     ssl->transform_in->ctx_inflate.avail_in = len_pre;
     ssl->transform_in->ctx_inflate.next_out = msg_post;
-    ssl->transform_in->ctx_inflate.avail_out = MBEDTLS_SSL_MAX_CONTENT_LEN;
+    ssl->transform_in->ctx_inflate.avail_out = MBEDTLS_SSL_BUFFER_LEN -
+                                               header_bytes;
 
     ret = inflate( &ssl->transform_in->ctx_inflate, Z_SYNC_FLUSH );
     if( ret != Z_OK )
@@ -2182,8 +2185,8 @@ static int ssl_decompress_buf( mbedtls_ssl_context *ssl )
         return( MBEDTLS_ERR_SSL_COMPRESSION_FAILED );
     }
 
-    ssl->in_msglen = MBEDTLS_SSL_MAX_CONTENT_LEN -
-                     ssl->transform_in->ctx_inflate.avail_out;
+    ssl->in_msglen = MBEDTLS_SSL_BUFFER_LEN -
+                     ssl->transform_in->ctx_inflate.avail_out - header_bytes;
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "after decompression: msglen = %d, ",
                    ssl->in_msglen ) );