diff --git a/worldeditadditions/utils/strings.lua b/worldeditadditions/utils/strings.lua
index 2751f4c..1caad5a 100644
--- a/worldeditadditions/utils/strings.lua
+++ b/worldeditadditions/utils/strings.lua
@@ -247,6 +247,7 @@ end
 -- @param	ms		float	The number of milliseconds to convert.
 -- @return	string	A human-readable string representing the input ms.
 function worldeditadditions.human_time(ms)
+	if type(ms) ~= "number" then return "unknown" end
 	local tokens = {
 		{ 31536000 * 1000, 'year' },
 		{ 2592000 * 1000, 'month' },