diff --git a/library/gcm.c b/library/gcm.c
index 104fda3..99036a0 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -296,7 +296,8 @@
     if( tag_len > 16 )
         return( POLARSSL_ERR_GCM_BAD_INPUT );
 
-    memcpy( tag, ctx->base_ectr, tag_len );
+    if( tag_len != 0 )
+        memcpy( tag, ctx->base_ectr, tag_len );
 
     if( orig_len || orig_add_len )
     {