diff --git a/src/calibre/constants.py b/src/calibre/constants.py
index bfcc009bc1..934b37de6a 100644
--- a/src/calibre/constants.py
+++ b/src/calibre/constants.py
@@ -22,9 +22,10 @@ isfreebsd = 'freebsd' in _plat
 isnetbsd = 'netbsd' in _plat
 isdragonflybsd = 'dragonfly' in _plat
 isbsd = isfreebsd or isnetbsd or isdragonflybsd
-islinux   = not(iswindows or isosx or isbsd)
+ishaiku = 'haiku1' in _plat
+islinux   = not(iswindows or isosx or isbsd or ishaiku)
 isfrozen  = hasattr(sys, 'frozen')
-isunix = isosx or islinux
+isunix = isosx or islinux or ishaiku
 isportable = os.environ.get('CALIBRE_PORTABLE_BUILD', None) is not None
 ispy3 = sys.version_info.major > 2
 isxp = iswindows and sys.getwindowsversion().major < 6