diff --git a/src/flexnbd.c b/src/flexnbd.c
index 23de499..5d35ec7 100644
--- a/src/flexnbd.c
+++ b/src/flexnbd.c
@@ -82,7 +82,7 @@ void params_serve(
 	out->bind_to.v4.sin_port = htobe16(out->bind_to.v4.sin_port);
 
 	out->filename = s_file;
-	out->filename_incomplete = xmalloc(strlen(s_file)+11);
+	out->filename_incomplete = xmalloc(strlen(s_file)+11+1);
 	strcpy(out->filename_incomplete, s_file);
 	strcpy(out->filename_incomplete + strlen(s_file), ".INCOMPLETE");
 }