diff --git a/lib/sprockets/utils.rb b/lib/sprockets/utils.rb
index 1233b78f7..bb0c68d71 100644
--- a/lib/sprockets/utils.rb
+++ b/lib/sprockets/utils.rb
@@ -106,7 +106,7 @@ def concat_javascript_sources(buf, source)
       source = source.encode(Encoding::UTF_32LE) unless source.ascii_only?
       return buf if string_end_with_semicolon?(source)
 
-      if whitespace = WHITESPACE_ORDINALS[buf[-1].ord]
+      if whitespace = WHITESPACE_ORDINALS[source[-1].ord]
         buf[-1] = ";#{whitespace}"
       else
         buf << ";"