diff --git a/stdlib/2.7/subprocess.pyi b/stdlib/2.7/subprocess.pyi
index 6053a4ede..7a42eff65 100644
--- a/stdlib/2.7/subprocess.pyi
+++ b/stdlib/2.7/subprocess.pyi
@@ -8,15 +8,15 @@ _FILE = Union[int, IO[Any]]
 
 # TODO force keyword arguments
 # TODO more keyword arguments (from Popen)
-def call(args: Sequence[str], *,
+def call(args: Union[str, Sequence[str]], *,
          stdin: _FILE = ..., stdout: _FILE = ..., stderr: _FILE = ...,
          shell: bool = ..., env: Mapping[str, str] = ...,
          cwd: str = ...) -> int: ...
-def check_call(args: Sequence[str], *,
+def check_call(args: Union[str, Sequence[str]], *,
                stdin: _FILE = ..., stdout: _FILE = ..., stderr: _FILE = ...,
                shell: bool = ..., env: Mapping[str, str] = ..., cwd: str = ...,
                close_fds: Sequence[_FILE] = ..., preexec_fn: Callable[[], Any] = ...) -> int: ...
-def check_output(args: Sequence[str], *,
+def check_output(args: Union[str, Sequence[str]], *,
                  stdin: _FILE = ..., stderr: _FILE = ...,
                  shell: bool = ..., universal_newlines: bool = ...,
                  env: Mapping[str, str] = ..., cwd: str = ...) -> str: ...
@@ -29,7 +29,7 @@ class CalledProcessError(Exception):
     cmd = ...  # type: str
     output = ...  # type: str # May be None
 
-    def __init__(self, returncode: int, cmd: str, output: str = ...) -> None: ...
+    def __init__(self, returncode: int, cmd: str, output: Optional[str] = ...) -> None: ...
 
 class Popen:
     stdin = ... # type: Optional[IO[Any]]
@@ -39,7 +39,7 @@ class Popen:
     returncode = 0
 
     def __init__(self,
-                 args: Sequence[str],
+                 args: Union[str, Sequence[str]],
                  bufsize: int = ...,
                  executable: str = ...,
                  stdin: _FILE = ...,
diff --git a/stdlib/3/subprocess.pyi b/stdlib/3/subprocess.pyi
index c606c33db..83d62cf9e 100644
--- a/stdlib/3/subprocess.pyi
+++ b/stdlib/3/subprocess.pyi
@@ -2,20 +2,20 @@
 
 # Based on http://docs.python.org/3.2/library/subprocess.html
 
-from typing import Sequence, Any, Mapping, Callable, Tuple, IO
+from typing import Sequence, Any, Mapping, Callable, Tuple, IO, Optional, Union
 
 # TODO force keyword arguments
 # TODO more keyword arguments
-def call(args: Sequence[str], *, stdin: Any = ..., stdout: Any = ...,
+def call(args: Union[str, Sequence[str]], *, stdin: Any = ..., stdout: Any = ...,
          stderr: Any = ..., shell: bool = ...,
          env: Mapping[str, str] = ...,
          cwd: str = ...) -> int: ...
-def check_call(args: Sequence[str], *, stdin: Any = ..., stdout: Any = ...,
+def check_call(args: Union[str, Sequence[str]], *, stdin: Any = ..., stdout: Any = ...,
                stderr: Any = ..., shell: bool = ...,
                env: Mapping[str, str] = ...,
                cwd: str = ...) -> int: ...
 # Return str/bytes
-def check_output(args: Sequence[str], *, stdin: Any = ..., stderr: Any = ...,
+def check_output(args: Union[str, Sequence[str]], *, stdin: Any = ..., stderr: Any = ...,
                  shell: bool = ..., universal_newlines: bool = ...,
                  env: Mapping[str, str] = ...,
                  cwd: str = ...) -> Any: ...
@@ -29,7 +29,8 @@ class CalledProcessError(Exception):
     cmd = ...  # type: str
     output = b'' # May be None
 
-    def __init__(self, returncode: int, cmd: str, output: str = ...) -> None: ...
+    def __init__(self, returncode: int, cmd: str, output: Optional[str],
+                 stderr: Optional[str] = ...) -> None: ...
 
 class Popen:
     stdin = ... # type: IO[Any]
@@ -39,7 +40,7 @@ class Popen:
     returncode = 0
 
     def __init__(self,
-                  args: Sequence[str],
+                  args: Union[str, Sequence[str]],
                   bufsize: int = ...,
                   executable: str = ...,
                   stdin: Any = ...,