diff --git a/tests/test_issues.py b/tests/test_issues.py
index e7547b9b..19d7a839 100644
--- a/tests/test_issues.py
+++ b/tests/test_issues.py
@@ -404,3 +404,13 @@ def test_issue434(self):
                     uproot.astable(
                         uproot.asdtype(fromdtype, todtype)), skipbytes=6))
         assert 486480 == hits['tdc'][0][0]
+
+    def test_issue438_accessing_memory_mapped_objects_outside_of_context_raises(self):
+        with uproot.open("tests/samples/issue434.root") as f:
+            a = f['KM3NET_EVENT']['KM3NET_EVENT']['KM3NETDAQ::JDAQPreamble'].array()
+            b = f['KM3NET_EVENT']['KM3NET_EVENT']['KM3NETDAQ::JDAQPreamble'].lazyarray()
+        assert 4 == len(a[0])
+        with pytest.raises(IOError):
+            len(b[0])
+
+
diff --git a/uproot/rootio.py b/uproot/rootio.py
index f5168fc3..ae1cb41e 100644
--- a/uproot/rootio.py
+++ b/uproot/rootio.py
@@ -362,6 +362,9 @@ def get(self, name, cycle=None):
             else:
                 raise _KeyError("not found: {0} with cycle {1}\n in file: {2}".format(repr(name), cycle, self._context.sourcepath))
 
+    def close(self):
+        self._context.source.close()
+
     def __contains__(self, name):
         try:
             self.get(name)
@@ -374,7 +377,7 @@ def __enter__(self, *args, **kwds):
         return self
 
     def __exit__(self, *args, **kwds):
-        pass
+        self.close()
 
 class _KeyError(KeyError):
     def __str__(self):
diff --git a/uproot/source/memmap.py b/uproot/source/memmap.py
index 55c21d89..379f1248 100644
--- a/uproot/source/memmap.py
+++ b/uproot/source/memmap.py
@@ -19,12 +19,19 @@ class MemmapSource(uproot.source.source.Source):
     def __init__(self, path):
         self.path = os.path.expanduser(path)
         self._source = numpy.memmap(self.path, dtype=numpy.uint8, mode="r")
+        self.closed = False
+
+    @property
+    def source(self):
+        if self.closed:
+            raise IOError("The file handler has already been closed.")
+        return self._source
 
     def parent(self):
         return self
 
     def size(self):
-        return len(self._source)
+        return len(self.source)
 
     def threadlocal(self):
         return self
@@ -33,17 +40,18 @@ def dismiss(self):
         pass
 
     def close(self):
-        self._source._mmap.close()
+        self.source._mmap.close()
+        self.closed = True
 
     def data(self, start, stop, dtype=None):
         # assert start >= 0
         # assert stop >= 0
         # assert stop >= start
 
-        if stop > len(self._source):
-            raise IndexError("indexes {0}:{1} are beyond the end of data source {2}".format(len(self._source), stop, repr(self.path)))
+        if stop > len(self.source):
+            raise IndexError("indexes {0}:{1} are beyond the end of data source {2}".format(len(self.source), stop, repr(self.path)))
 
         if dtype is None:
-            return self._source[start:stop]
+            return self.source[start:stop]
         else:
-            return self._source[start:stop].view(dtype)
+            return self.source[start:stop].view(dtype)