diff --git a/ChangeLog b/ChangeLog
index ed6f221..d64ce68 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -17,6 +17,8 @@
    * Fixed the sample applications gen_key.c, cert_req.c and cert_write.c for
      builds where the configuration POLARSSL_PEM_WRITE_C is not defined. Found
      by inestlerode. #559.
+   * Fix documentation and implementation missmatch for function arguments of
+     mbedtls_gcm_finish(). Found by cmiatpaar.
 
 = mbed TLS 1.3.17 branch 2016-06-28
 
diff --git a/include/polarssl/gcm.h b/include/polarssl/gcm.h
index 5a94722..3326919 100644
--- a/include/polarssl/gcm.h
+++ b/include/polarssl/gcm.h
@@ -186,8 +186,8 @@
  *                  16 bytes.
  *
  * \param ctx       GCM context
- * \param tag       buffer for holding the tag (may be NULL if tag_len is 0)
- * \param tag_len   length of the tag to generate
+ * \param tag       buffer for holding the tag
+ * \param tag_len   length of the tag to generate (must be at least 4)
  *
  * \return          0 if successful or POLARSSL_ERR_GCM_BAD_INPUT
  */
diff --git a/library/gcm.c b/library/gcm.c
index 83f2fd2..33306e5 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -410,8 +410,7 @@
     if( tag_len > 16 || tag_len < 4 )
         return( POLARSSL_ERR_GCM_BAD_INPUT );
 
-    if( tag_len != 0 )
-        memcpy( tag, ctx->base_ectr, tag_len );
+    memcpy( tag, ctx->base_ectr, tag_len );
 
     if( orig_len || orig_add_len )
     {