diff --git a/httpie/downloads.py b/httpie/downloads.py
index 3ce8dbc8..18263785 100644
--- a/httpie/downloads.py
+++ b/httpie/downloads.py
@@ -149,6 +149,7 @@ class Download(object):
         :type progress_file: file
 
         """
+        assert output_file, output_file
         self._output_file = output_file
         self._resume = resume
         self._resumed_from = 0
@@ -208,8 +209,11 @@ class Download(object):
 
             else:
                 self._resumed_from = 0
-                self._output_file.seek(0)
-                self._output_file.truncate()
+                try:
+                    self._output_file.seek(0)
+                    self._output_file.truncate()
+                except IOError:
+                    pass  # stdout
         else:
             # TODO: Should the filename be taken from response.history[0].url?
             # Output file not specified. Pick a name that doesn't exist yet.
diff --git a/httpie/input.py b/httpie/input.py
index 2f0f1352..d8f5d77a 100644
--- a/httpie/input.py
+++ b/httpie/input.py
@@ -140,13 +140,23 @@ class Parser(ArgumentParser):
         Modify `env.stdout` and `env.stdout_isatty` based on args, if needed.
 
         """
+        if not self.env.stdout_isatty and self.args.output_file:
+            self.error('Cannot use --output, -o with redirected output.')
+
+        # FIXME: Come up with a cleaner solution.
         if self.args.download:
+
+            if not self.env.stdout_isatty:
+                # Use stdout as tge download output file.
+                self.args.output_file = self.env.stdout
+
             # With `--download`, we write everything that would normally go to
             # `stdout` to `stderr` instead. Let's replace the stream so that
             # we don't have to use many `if`s throughout the codebase.
             # The response body will be treated separately.
             self.env.stdout = self.env.stderr
             self.env.stdout_isatty = self.env.stderr_isatty
+
         elif self.args.output_file:
             # When not `--download`ing, then `--output` simply replaces
             # `stdout`. The file is opened for appending, which isn't what