diff --git a/include/boost/config/stdlib/msl.hpp b/include/boost/config/stdlib/msl.hpp
index 4c6bad4a..f8ad3d9a 100644
--- a/include/boost/config/stdlib/msl.hpp
+++ b/include/boost/config/stdlib/msl.hpp
@@ -30,7 +30,9 @@
 
 #if defined(__MSL__) && (__MSL__ >= 0x5000)
 #  define BOOST_HAS_STDINT_H
-#  define BOOST_HAS_UNISTD_H
+#  if !defined(__PALMOS_TRAPS__)
+#    define BOOST_HAS_UNISTD_H
+#  endif
    // boilerplate code:
 #  include <boost/config/posix_features.hpp>
 #endif