diff --git a/setup/sfntly.py b/setup/sfntly.py
index 60d7808d5f..79a25a53f0 100644
--- a/setup/sfntly.py
+++ b/setup/sfntly.py
@@ -45,9 +45,9 @@ class SfntlyBuilderMixin(object):
         if iswindows:
             self.sfntly_cflags += [
                     '-D_UNICODE', '-DUNICODE',
-            ] + shlex.split('/Zi /nologo /W4 /WX /O2 /Ob2 /Oy /GF /Gm- /MT /GS /Gy '
-            '/fp:precise /Zc:wchar_t /Zc:forScope /GR-')
+            ] + shlex.split('/W4 /WX /Gm- /Gy /GR-')
         else:
+            # Possibly add -fno-inline (slower, but more robust)
             self.sfntly_cflags += [
                     '-Werror',
                     '-fno-exceptions',
@@ -59,6 +59,11 @@ class SfntlyBuilderMixin(object):
 
     def __call__(self, obj_dir, compiler, linker, builder, cflags, ldflags):
         self.sfntly_build_dir = os.path.join(obj_dir, 'sfntly')
+        if '/Ox' in cflags:
+            cflags.remove('/Ox')
+        if '-O3' in cflags:
+            cflags.remove('-O3')
+        cflags.insert(0, '/O2' if iswindows else '-O2')
 
         groups = []
         all_headers = set()
diff --git a/src/calibre/test_build.py b/src/calibre/test_build.py
index d6b3c9a400..2b00d63163 100644
--- a/src/calibre/test_build.py
+++ b/src/calibre/test_build.py
@@ -37,6 +37,11 @@ def test_freetype():
     test()
     print ('FreeType OK!')
 
+def test_sfntly():
+    from calibre.utils.fonts.subset import test
+    test()
+    print ('sfntly OK!')
+
 def test_winutil():
     from calibre.devices.scanner import win_pnp_drives
     matches = win_pnp_drives.scanner()
@@ -115,6 +120,7 @@ def test():
     test_plugins()
     test_lxml()
     test_freetype()
+    test_sfntly()
     test_sqlite()
     test_qt()
     test_imaging()
diff --git a/src/calibre/utils/fonts/subset.py b/src/calibre/utils/fonts/subset.py
index 30d0109b30..41f0365cdf 100644
--- a/src/calibre/utils/fonts/subset.py
+++ b/src/calibre/utils/fonts/subset.py
@@ -93,6 +93,11 @@ def test_mem():
     for i in xrange(3): gc.collect()
     print ('Leaked memory per call:', (memory() - start_mem)/calls*1024, 'KB')
 
+def test():
+    raw = P('fonts/liberation/LiberationSerif-Regular.ttf', data=True)
+    sf, old_stats, new_stats = subset(raw, set(('a', 'b', 'c')), ())
+    if len(sf) > 0.3 * len(raw):
+        raise Exception('Subsetting failed')
 
 def main(args):
     import sys, time