diff --git a/CmdLine.lua b/CmdLine.lua
index a9aa6609..a53a0b3f 100644
--- a/CmdLine.lua
+++ b/CmdLine.lua
@@ -19,7 +19,7 @@ local function block_txt(str, pre_pad, block_sz)
    local start_i = 0
    local end_i = 0
 
-   while end_i do
+   while end_i and str do
       -- start search for text ahead of previous last position
       start_i = end_i + 1
       -- newline if the string already has text