diff --git a/docker-compose/tango-prometheus-exporter/code/tango-prometheus-client.py b/docker-compose/tango-prometheus-exporter/code/tango-prometheus-client.py
index aaffe96637aefb3aafd83acc2deb7a1ce43915fd..d171d9219e2bb3043f6c8be0d91a32bd8a9fb807 100644
--- a/docker-compose/tango-prometheus-exporter/code/tango-prometheus-client.py
+++ b/docker-compose/tango-prometheus-exporter/code/tango-prometheus-client.py
@@ -69,8 +69,7 @@ class CustomCollector(object):
         self.policy = ArchiverPolicy(config)
         self.proxy_timeout = proxy_timeout
 
-    @staticmethod
-    def _to_metric(dev, attr_info, x, y, idx, value):
+    def _to_metric(self, dev, attr_info, x, y, idx, value):
         """ Convert the given values to a (labels, value) pair, used to construct a Metric. """
 
         if attr_info.data_type in [ArgType.DevShort, ArgType.DevLong, ArgType.DevUShort, ArgType.DevULong, ArgType.DevLong64, ArgType.DevULong64, ArgType.DevInt, ArgType.DevFloat, ArgType.DevDouble]: