diff --git lib/roo/utils.rb lib/roo/utils.rb
index dcaedde..bd56139 100644
--- lib/roo/utils.rb
+++ lib/roo/utils.rb
@@ -9,6 +9,8 @@ module Roo
       num_only = false
 
       s.each_byte do |b|
+        # In the case that a range is supplied, stop at the : delimiter character
+        break if b == 58
         if !num_only && (index = char_index(b))
           letter_num *= 26
           letter_num += index