Use a specific index for the _count attribute. This might fix issue…

…s with indexing errors

brian2/monitors/spikemonitor.py

@@ -56,8 +56,10 @@ def __init__(self, source, record=True, when='end', order=0,
                                          dtype=np.int32, constant_size=False)
         self.variables.add_dynamic_array('t', size=0, unit=second,
                                          constant_size=False)
+        self.variables.add_arange('_source_i', size=len(source))
         self.variables.add_array('_count', size=len(source), unit=Unit(1),
-                                 dtype=np.int32)
+                                 dtype=np.int32, read_only=True,
+                                 index='_source_i')
         self.variables.add_constant('_source_start', Unit(1), start)
         self.variables.add_constant('_source_stop', Unit(1), stop)
         self.variables.add_attribute_variable('N', unit=Unit(1), obj=self,