--- a/taskcluster/taskgraph/transforms/base.py
+++ b/taskcluster/taskgraph/transforms/base.py
@@ -124,20 +124,26 @@ def get_keyed_by(item, field, item_name,
     keyed_by = value.keys()[0]
     if len(value) > 1 or not keyed_by.startswith('by-'):
         return value
 
     values = value[keyed_by]
     keyed_by = keyed_by[3:]  # strip 'by-' off the keyed-by field name
     if item[keyed_by] in values:
         return values[item[keyed_by]]
-    for k in values.keys():
-        if re.match(k, item[keyed_by]):
-            return values[k]
+
+    matches = [(k,v )for k, v in values.iteritems() if re.match(k, item[keyed_by])]
+    if len(matches) > 1:
+        raise Exception(
+            "Multiple matching values for {} {!r} found while determining item {} in {}".format(
+                keyed_by, item[keyed_by], field, item_name))
+    elif matches:
+        return matches[0][1]
+
     if 'default' in values:
         return values['default']
     for k in item[keyed_by], 'default':
         if k in values:
             return values[k]
     else:
         raise Exception(
-            "Neither {} {} nor 'default' found while determining item {} in {}".format(
+            "No {} matching {!r} nor 'default' found while determining item {} in {}".format(
                 keyed_by, item[keyed_by], field, item_name))