diff --git a/prometheus_client/__init__.py b/prometheus_client/__init__.py
index c650c6d1..416c25c7 100644
--- a/prometheus_client/__init__.py
+++ b/prometheus_client/__init__.py
@@ -42,7 +42,7 @@ def register(self, collector):
     with self._lock:
       self._collectors.add(collector)
 
-  def unregister(self, metric):
+  def unregister(self, collector):
     '''Remove a collector from the registry.'''
     with self._lock:
       self._collectors.remove(collector)
@@ -102,7 +102,7 @@ def __init__(self, wrappedClass, labelnames, **kwargs):
 
     for l in labelnames:
       if l.startswith('__'):
-        raise InvalidLabelName(l)
+        raise ValueError('Invalid label metric name: ' + l)
 
   def labels(self, *labelvalues):
     '''Return the child for the given labelset.'''