diff --git a/core/io/encodings/utf16/utf16.factor b/core/io/encodings/utf16/utf16.factor
index 0660ddfd77..24491a7a26 100644
--- a/core/io/encodings/utf16/utf16.factor
+++ b/core/io/encodings/utf16/utf16.factor
@@ -2,7 +2,8 @@
 ! See http://factorcode.org/license.txt for BSD license.
 USING: accessors byte-arrays combinators io io.binary
 io.encodings kernel math math.private namespaces sbufs
-sequences sequences.private splitting strings.private vectors ;
+sequences sequences.private splitting strings strings.private
+vectors ;
 IN: io.encodings.utf16
 
 SINGLETON: utf16be
@@ -115,13 +116,13 @@ M: utf16le encode-char ( char stream encoding -- )
 
 M: utf16le encode-string
     drop
-    over aux>>
+    over dup string? [ aux>> ] [ drop t ] if
     [ [ char>utf16le ] curry each ]
     [ ascii-string>utf16le ] if ;
 
 M: utf16be encode-string
     drop
-    over aux>>
+    over dup string? [ aux>> ] [ drop t ] if
     [ [ char>utf16be ] curry each ]
     [ ascii-string>utf16be ] if ;
 
diff --git a/core/io/encodings/utf8/utf8.factor b/core/io/encodings/utf8/utf8.factor
index 28ce4899f0..d07893c0ef 100644
--- a/core/io/encodings/utf8/utf8.factor
+++ b/core/io/encodings/utf8/utf8.factor
@@ -1,7 +1,7 @@
 ! Copyright (C) 2006, 2008 Daniel Ehrenberg.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: accessors combinators io io.encodings
-io.encodings.private kernel math math.order sequences ;
+io.encodings.private kernel math math.order sequences strings ;
 IN: io.encodings.utf8
 
 ! Decoding UTF-8
@@ -86,7 +86,7 @@ M: utf8 encode-char
 
 M: utf8 encode-string
     drop
-    over aux>>
+    over dup string? [ aux>> ] [ drop t ] if
     [ [ char>utf8 ] curry each ]
     [ [ string>byte-array-fast ] dip stream-write ] if ;