diff --git a/src/MD_STD_IN.php b/src/MD_STD_IN.php
index 54b060a..5b21a55 100644
--- a/src/MD_STD_IN.php
+++ b/src/MD_STD_IN.php
@@ -411,7 +411,7 @@ final class MD_STD_IN {
         }
 
         // Remove hyphens
-        $input = trim(strtr($input, ["-" => "", "–" => ""]));
+        $input = trim(strtr($input, ["-" => "", "–" => "", " " => ""]));
 
         // ISBN 10
         if (\mb_strlen($input) === 10) {