diff --git a/sphinx/environment.py b/sphinx/environment.py
index 6ad28ec8a..bf255165e 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -630,6 +630,8 @@ class BuildEnvironment:
 
         class SphinxSourceClass(FileInput):
             def decode(self_, data):
+                if isinstance(data, unicode):
+                    return data
                 return data.decode(self_.encoding, 'sphinx')
 
             def read(self_):