diff --git a/src/nexusformat/nexus/plot.py b/src/nexusformat/nexus/plot.py
index e024299..d6842fc 100644
--- a/src/nexusformat/nexus/plot.py
+++ b/src/nexusformat/nexus/plot.py
@@ -194,8 +194,8 @@ def plot(self, data_group, fmt=None, xmin=None, xmax=None,
                     fmt = kwargs.pop('marker')
                 else:
                     fmt = 'o'
-                if hasattr(signal, 'units'):
-                    if not errors and signal.units == 'counts':
+                if 'units' in signal.attrs:
+                    if not errors and signal.attrs['units'] == 'counts':
                         errors = NXfield(np.sqrt(data))
                 if errors:
                     ebars = errors.nxdata