Fixed broken error message
@@ -373,7 +373,7 @@ class aggregate(ResamplingOperation):
if not dims:
raise ValueError("Aggregation column %s not found on %s element. "
"Ensure the aggregator references an existing "
- "dimension.")
+ "dimension." % (column,element))
if isinstance(agg_fn, ds.count_cat):
name = '%s Count' % agg_fn.column
vdims = [dims[0](column)]
