diff --git a/bin/population_template b/bin/population_template
index a79b31e169..b44506ebf3 100755
--- a/bin/population_template
+++ b/bin/population_template
@@ -565,7 +565,7 @@ def parse_input_files(in_files, mask_files, contrasts, f_agg_weight=None, whites
         agg_weights = dict((row[0].strip(), row[1].strip()) for row in reader)
     pref = '^' + re.escape(get_common_prefix(list(agg_weights.keys())))
     suff = re.escape(get_common_postfix(list(agg_weights.keys()))) + '$'
-    agg_weights = {re.sub(suff, '', re.sub(pref, '', key)):agg_weights[key] for key in agg_weights.keys()}
+    agg_weights = {re.sub(suff, '', re.sub(pref, '', item[0])):item[1] for item in agg_weights.items()}
     for inp in inputs:
       if inp.uid not in agg_weights:
         raise MRtrixError('aggregation weight not found for input "%s"' % inp.uid)