Index: subprocess.py
===================================================================
--- subprocess.py	(revision 64516)
+++ subprocess.py	(working copy)
@@ -656,7 +656,9 @@
             stdout = None
             stderr = None
             if self.stdin:
-                if input:
+                if input is not None:
+                    if isinstance(input, str):
+                        input = input.encode()
                     self.stdin.write(input)
                 self.stdin.close()
             elif self.stdout: