diff --git a/ChangeLog b/ChangeLog
index c77c58f..fc08e89 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -38,6 +38,8 @@
    * Fixed the sample applications gen_key.c, cert_req.c and cert_write.c for
      builds where the configuration MBEDTLS_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.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index 6743ac9..1b77aae 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -190,8 +190,8 @@
  *                  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 MBEDTLS_ERR_GCM_BAD_INPUT
  */
diff --git a/library/gcm.c b/library/gcm.c
index aaacf97..f1210c5 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -415,8 +415,7 @@
     if( tag_len > 16 || tag_len < 4 )
         return( MBEDTLS_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 )
     {