diff --git a/fire/core.py b/fire/core.py
index 6fd1bf7..4546b27 100644
--- a/fire/core.py
+++ b/fire/core.py
@@ -164,7 +164,8 @@ def Fire(component=None, command=None, name=None, serialize=None):
     raise FireExit(0, component_trace)
 
   # The command succeeded normally; print the result.
-  _PrintResult(component_trace, verbose=component_trace.verbose, serialize=serialize)
+  _PrintResult(
+      component_trace, verbose=component_trace.verbose, serialize=serialize)
   result = component_trace.GetResult()
   return result
 
@@ -247,11 +248,12 @@ def _PrintResult(component_trace, verbose=False, serialize=None):
   # and move serialization to its own module.
   result = component_trace.GetResult()
 
-  # Allow users to modify the return value of the component and provide 
+  # Allow users to modify the return value of the component and provide
   # custom formatting.
   if serialize:
     if not callable(serialize):
-      raise FireError("serialize argument {} must be empty or callable.".format(serialize))
+      raise FireError(
+          'The argument `serialize` must be empty or callable:', serialize)
     result = serialize(result)
 
   if value_types.HasCustomStr(result):
diff --git a/fire/core_test.py b/fire/core_test.py
index a0576ee..0d11467 100644
--- a/fire/core_test.py
+++ b/fire/core_test.py
@@ -199,21 +199,22 @@ def serialize(x):
       if isinstance(x, list):
         return ', '.join(str(xi) for xi in x)
       if isinstance(x, dict):
-        return ', '.join('{}={!r}'.format(k, v) for k, v in x.items())
+        return ', '.join('{}={!r}'.format(k, v) for k, v in sorted(x.items()))
       if x == 'special':
         return ['SURPRISE!!', "I'm a list!"]
       return x
 
     ident = lambda x: x
-    
+
     with self.assertOutputMatches(stdout='a, b', stderr=None):
-      result = core.Fire(ident, command=['[a,b]'], serialize=serialize)
+      _ = core.Fire(ident, command=['[a,b]'], serialize=serialize)
     with self.assertOutputMatches(stdout='a=5, b=6', stderr=None):
-      result = core.Fire(ident, command=['{a:5,b:6}'], serialize=serialize)
+      _ = core.Fire(ident, command=['{a:5,b:6}'], serialize=serialize)
     with self.assertOutputMatches(stdout='asdf', stderr=None):
-      result = core.Fire(ident, command=['asdf'], serialize=serialize)
-    with self.assertOutputMatches(stdout="SURPRISE!!\nI'm a list!\n", stderr=None):
-      result = core.Fire(ident, command=['special'], serialize=serialize)
+      _ = core.Fire(ident, command=['asdf'], serialize=serialize)
+    with self.assertOutputMatches(
+        stdout="SURPRISE!!\nI'm a list!\n", stderr=None):
+      _ = core.Fire(ident, command=['special'], serialize=serialize)
     with self.assertRaises(core.FireError):
       core.Fire(ident, command=['asdf'], serialize=55)