diff --git a/lib/vpim/rfc2425.rb b/lib/vpim/rfc2425.rb
index f2a6150..9fa1be7 100644
--- a/lib/vpim/rfc2425.rb
+++ b/lib/vpim/rfc2425.rb
@@ -290,8 +290,9 @@ def Vpim.decode_text_list(value, sep = ',') # :nodoc:
   # paramtext  = *SAFE-CHAR
   # quoted-string      = DQUOTE *QSAFE-CHAR DQUOTE
   def Vpim.encode_paramtext(value)
-    case value
-    when %r{\A#{Bnf::SAFECHAR}*\z}
+    bin_value = value.dup.force_encoding('ASCII-8BIT')
+    case bin_value
+    when %r{\A#{Bnf::SAFECHAR}*\z}n
       value
     else
       raise Vpim::Unencodeable, "paramtext #{value.inspect}"
@@ -299,10 +300,11 @@ def Vpim.encode_paramtext(value)
   end
 
   def Vpim.encode_paramvalue(value)
-    case value
-    when %r{\A#{Bnf::SAFECHAR}*\z}
+    bin_value = value.dup.force_encoding('ASCII-8BIT')
+    case bin_value
+    when %r{\A#{Bnf::SAFECHAR}*\z}n
       value
-    when %r{\A#{Bnf::QSAFECHAR}*\z}
+    when %r{\A#{Bnf::QSAFECHAR}*\z}n
       '"' + value + '"'
     else
       raise Vpim::Unencodeable, "param-value #{value.inspect}"