diff --git a/lib/net/imap/command_data.rb b/lib/net/imap/command_data.rb
index 1fd0dc30..cab59a97 100644
--- a/lib/net/imap/command_data.rb
+++ b/lib/net/imap/command_data.rb
@@ -459,7 +459,7 @@ def valid_nstring?(str)
       # coerces using +to_s+
       def string(str)
         str = str.to_s
-        if str =~ LITERAL_REGEX
+        if str.b.match?(LITERAL_REGEX)
           Literal.new(str)
         else
           QuotedString.new(str)