diff --git a/britney2/excuse.py b/britney2/excuse.py
index e1e107c..d7ca525 100644
--- a/britney2/excuse.py
+++ b/britney2/excuse.py
@@ -60,7 +60,7 @@ class Excuse(object):
         self.deps = {}
         self.sane_deps = []
         self.break_deps = []
-        self.unsat_deps = {}
+        self.unsat_deps = defaultdict(set)
         self.bugs = []
         self.newbugs = set()
         self.oldbugs = set()
@@ -126,10 +126,7 @@ class Excuse(object):
 
     def add_unsatisfiable_dep(self, signature, arch):
         """Add an unsatisfiable dependency"""
-        if arch not in self.unsat_deps:
-            self.unsat_deps[arch] = []
-        if signature not in self.unsat_deps[arch]:
-            self.unsat_deps[arch].append(signature)
+        self.unsat_deps[arch].add(signature)
 
     def invalidate_dep(self, name):
         """Invalidate dependency"""
@@ -266,11 +263,7 @@ class Excuse(object):
             if break_deps:
                 dep_data['unimportant-dependencies'] = sorted(break_deps)
             if self.unsat_deps:
-                dep_data['unsatisfiable-dependencies'] = unsatisfiable = {}
-                for arch in self.unsat_deps:
-                    unsatisfiable[arch] = []
-                    for sig in self.unsat_deps[arch]:
-                        unsatisfiable[arch].append("%s" % sig)
+                dep_data['unsatisfiable-dependencies'] = self.unsat_deps
         if self.needs_approval:
             status = 'not-approved'
             for h in self.hints:
diff --git a/tests/test_yaml.py b/tests/test_yaml.py
index 86063e5..a8a9a86 100755
--- a/tests/test_yaml.py
+++ b/tests/test_yaml.py
@@ -81,7 +81,6 @@ class YamlTest(TestBase):
                 do = time.time() - (60 * 60 * 24 * daysold)
                 f.write('%s %s %d' % (pkg, fields['Version'], do))
 
-
         # Set up sourceppa cache for testing
         sourceppa_path = os.path.join(self.data.dirs[True], 'SourcePPA')
         with open(sourceppa_path, 'w', encoding='utf-8') as sourceppa:
@@ -124,7 +123,7 @@ class YamlTest(TestBase):
                6)
 
         excuse = self.do_test([pkg])
-        self.assertIn('datefudge', excuse['libc6']['dependencies']['unsatisfiable-dependencies']['amd64'][0])
+        self.assertIn('datefudge', list(excuse['libc6']['dependencies']['unsatisfiable-dependencies']['amd64'])[0])