diff --git a/prometheus_client/core.py b/prometheus_client/core.py
index cb1e7c5b..b05bf645 100644
--- a/prometheus_client/core.py
+++ b/prometheus_client/core.py
@@ -505,11 +505,11 @@ class _MmapedDict(object):
     Not thread safe.
     """
     def __init__(self, filename, read_mode=False):
-        self._f = open(filename, 'a+b')
+        self._f = open(filename, 'rb' if read_mode else 'a+b')
         if os.fstat(self._f.fileno()).st_size == 0:
             self._f.truncate(_INITIAL_MMAP_SIZE)
         self._capacity = os.fstat(self._f.fileno()).st_size
-        self._m = mmap.mmap(self._f.fileno(), self._capacity)
+        self._m = mmap.mmap(self._f.fileno(), self._capacity, access=mmap.ACCESS_READ if read_mode else mmap.ACCESS_WRITE)
 
         self._positions = {}
         self._used = _unpack_integer(self._m, 0)[0]
diff --git a/prometheus_client/exposition.py b/prometheus_client/exposition.py
index 1ebeba29..29494d08 100644
--- a/prometheus_client/exposition.py
+++ b/prometheus_client/exposition.py
@@ -37,7 +37,7 @@ def make_wsgi_app(registry=core.REGISTRY):
     def prometheus_app(environ, start_response):
         params = parse_qs(environ.get('QUERY_STRING', ''))
         r = registry
-        encoder, content_type = choose_encoder(environ.get['HTTP_ACCEPT'])
+        encoder, content_type = choose_encoder(environ.get('HTTP_ACCEPT'))
         if 'name[]' in params:
             r = r.restricted_registry(params['name[]'])
         output = encoder(r)