diff --git a/Lib/distutils/command/build_ext.py b/Lib/distutils/command/build_ext.py
index 54d4846..9bb7e77 100644
--- a/Lib/distutils/command/build_ext.py
+++ b/Lib/distutils/command/build_ext.py
@@ -309,7 +309,7 @@
 
 
     def get_source_files (self):
-
+        self.check_extension_list()
         filenames = []
 
         # Wouldn't it be neat if we knew the names of header files too...