diff --git a/CHANGELOG.md b/CHANGELOG.md
index d770aa4..ab80cc1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 0.3.11 (2019-08-18)
+## 0.3.12 (2019-08-18)
 * Fix: use `env` format from stdin
 
 ## 0.3.10 (2019-06-07)
diff --git a/README.md b/README.md
index 5461a8a..b34a436 100644
--- a/README.md
+++ b/README.md
@@ -127,12 +127,17 @@ Or even read environment variables from a file:
 
     $ j2 --format=env config.j2 data.env
 
+Or pipe it: (note that you'll have to use the "-" in this particular case):
+
+    $ j2 --format=env config.j2 - < data.env
+
     
 # Reference
 `j2` accepts the following arguments:
 
 * `template`: Jinja2 template file to render
-* `data`: (optional) path to the data used for rendering. The default is `-`: use stdin
+* `data`: (optional) path to the data used for rendering.
+    The default is `-`: use stdin. Specify it explicitly when using env!
 
 Options:
 
@@ -166,7 +171,7 @@ Render directly from the current environment variable values:
 
     $ j2 config.j2
 
-Or alternatively, read the values from a file:
+Or alternatively, read the values from a dotenv file:
 
 ```
 NGINX_HOSTNAME=localhost
@@ -179,7 +184,9 @@ And render with:
     $ j2 config.j2 data.env
     $ env | j2 --format=env config.j2
 
-This is especially useful with Docker to link containers together.
+If you're going to pipe a dotenv file into `j2`, you'll need to use "-" as the second argument to explicitly:
+
+    $ j2 config.j2 - < data.env
 
 ### ini
 INI data input format.
diff --git a/j2cli/cli.py b/j2cli/cli.py
index 883ec73..fae229a 100644
--- a/j2cli/cli.py
+++ b/j2cli/cli.py
@@ -121,12 +121,12 @@ def render_command(cwd, environ, stdin, argv):
     parser.add_argument('--undefined', action='store_true', dest='undefined', help='Allow undefined variables to be used in templates (no error will be raised)')
     parser.add_argument('-o', metavar='outfile', dest='output_file', help="Output to a file instead of stdout")
     parser.add_argument('template', help='Template file to process')
-    parser.add_argument('data', nargs='?', default='-', help='Input data path')
+    parser.add_argument('data', nargs='?', default=None, help='Input data file path; "-" to use stdin')
     args = parser.parse_args(argv)
 
     # Input: guess format
     if args.format == '?':
-        if args.data == '-':
+        if args.data is None or args.data == '-':
             args.format = 'env'
         else:
             args.format = {
@@ -138,10 +138,27 @@ def render_command(cwd, environ, stdin, argv):
             }[os.path.splitext(args.data)[1]]
 
     # Input: data
-    if args.data == '-' and args.format == 'env' and (stdin is None or stdin.isatty()):
-        input_data_f = None
+    # We always expect a file;
+    # unless the user wants 'env', and there's no input file provided.
+    if args.format == 'env':
+        # With the "env" format, if no dotenv filename is provided, we have two options:
+        # either the user wants to use the current environment, or he's feeding a dotenv file at stdin.
+        # Depending on whether we have data at stdin, we'll need to choose between the two.
+        #
+        # The problem is that in Linux, you can't reliably determine whether there is any data at stdin:
+        # some environments would open the descriptor even though they're not going to feed any data in.
+        # That's why many applications would ask you to explicitly specify a '-' when stdin should be used.
+        #
+        # And this is what we're going to do here as well.
+        # The script, however, would give the user a hint that they should use '-'
+        if args.data == '-':
+            input_data_f = stdin
+        elif args.data == None:
+            input_data_f = None
+        else:
+            input_data_f = open(args.data)
     else:
-        input_data_f = stdin if args.data == '-' else open(args.data)
+        input_data_f = stdin if args.data is None or args.data == '-' else open(args.data)
 
     # Python 2: Encode environment variables as unicode
     if sys.version_info[0] == 2 and args.format == 'env':
@@ -183,7 +200,25 @@ def render_command(cwd, environ, stdin, argv):
     renderer.register_tests(customize.extra_tests())
 
     # Render
-    result = renderer.render(args.template, context)
+    try:
+        result = renderer.render(args.template, context)
+    except jinja2.exceptions.UndefinedError as e:
+        # When there's data at stdin, tell the user they should use '-'
+        try:
+            stdin_has_data = stdin is not None and not stdin.isatty()
+            if args.format == 'env' and args.data == None and stdin_has_data:
+                extra_info = (
+                    "\n\n"
+                    "If you're trying to pipe a .env file, please run me with a '-' as the data file name:\n"
+                    "$ {cmd} {argv} -".format(cmd=os.path.basename(sys.argv[0]), argv=' '.join(sys.argv[1:]))
+                )
+                e.args = (e.args[0] + extra_info,) + e.args[1:]
+        except:
+            # The above code is so optional that any, ANY, error, is ignored
+            pass
+
+        # Proceed
+        raise
 
     # -o
     if args.output_file:
@@ -199,11 +234,14 @@ def render_command(cwd, environ, stdin, argv):
 
 def main():
     """ CLI Entry point """
-    output = render_command(
-        os.getcwd(),
-        os.environ,
-        sys.stdin,
-        sys.argv[1:]
-    )
+    try:
+        output = render_command(
+            os.getcwd(),
+            os.environ,
+            sys.stdin,
+            sys.argv[1:]
+        )
+    except SystemExit:
+        return 1
     outstream = getattr(sys.stdout, 'buffer', sys.stdout)
     outstream.write(output)
diff --git a/j2cli/context.py b/j2cli/context.py
index 2b14bc8..f8035d4 100644
--- a/j2cli/context.py
+++ b/j2cli/context.py
@@ -102,7 +102,7 @@ def _parse_env(data_string):
 
         $ j2 config.j2
 
-    Or alternatively, read the values from a file:
+    Or alternatively, read the values from a dotenv file:
 
     ```
     NGINX_HOSTNAME=localhost
@@ -115,7 +115,9 @@ def _parse_env(data_string):
         $ j2 config.j2 data.env
         $ env | j2 --format=env config.j2
 
-    This is especially useful with Docker to link containers together.
+    If you're going to pipe a dotenv file into `j2`, you'll need to use "-" as the second argument to explicitly:
+
+        $ j2 config.j2 - < data.env
     """
     # Parse
     if isinstance(data_string, basestring):
diff --git a/misc/_doc/README.md.j2 b/misc/_doc/README.md.j2
index 8f7fc06..d068d7f 100644
--- a/misc/_doc/README.md.j2
+++ b/misc/_doc/README.md.j2
@@ -127,12 +127,17 @@ Or even read environment variables from a file:
 
     $ j2 --format=env config.j2 data.env
 
+Or pipe it: (note that you'll have to use the "-" in this particular case):
+
+    $ j2 --format=env config.j2 - < data.env
+
     
 # Reference
 `j2` accepts the following arguments:
 
 * `template`: Jinja2 template file to render
-* `data`: (optional) path to the data used for rendering. The default is `-`: use stdin
+* `data`: (optional) path to the data used for rendering.
+    The default is `-`: use stdin. Specify it explicitly when using env!
 
 Options:
 
diff --git a/setup.py b/setup.py
index 68d46fb..71410d6 100755
--- a/setup.py
+++ b/setup.py
@@ -26,7 +26,7 @@
 
 setup(
     name='j2cli',
-    version='0.3.11',
+    version='0.3.12b',
     author='Mark Vartanyan',
     author_email='kolypto@gmail.com',
 
diff --git a/tests/render-test.py b/tests/render-test.py
index d0f44f4..791764a 100644
--- a/tests/render-test.py
+++ b/tests/render-test.py
@@ -70,6 +70,7 @@ def test_ini(self):
         self._testme_std(['--format=ini', 'resources/nginx.j2', 'resources/data.ini'])
         # Stdin
         self._testme_std(['--format=ini', 'resources/nginx.j2'], stdin=open('resources/data.ini'))
+        self._testme_std(['--format=ini', 'resources/nginx.j2', '-'], stdin=open('resources/data.ini'))
 
     def test_json(self):
         # Filename
@@ -78,6 +79,7 @@ def test_json(self):
         self._testme_std(['--format=json', 'resources/nginx.j2', 'resources/data.json'])
         # Stdin
         self._testme_std(['--format=json', 'resources/nginx.j2'], stdin=open('resources/data.json'))
+        self._testme_std(['--format=json', 'resources/nginx.j2', '-'], stdin=open('resources/data.json'))
 
     def test_yaml(self):
         try:
@@ -92,19 +94,24 @@ def test_yaml(self):
         self._testme_std(['--format=yaml', 'resources/nginx.j2', 'resources/data.yml'])
         # Stdin
         self._testme_std(['--format=yaml', 'resources/nginx.j2'], stdin=open('resources/data.yml'))
+        self._testme_std(['--format=yaml', 'resources/nginx.j2', '-'], stdin=open('resources/data.yml'))
 
     def test_env(self):
         # Filename
-        self._testme_std(['resources/nginx-env.j2', 'resources/data.env'])
+        self._testme_std(['--format=env', 'resources/nginx-env.j2', 'resources/data.env'])
+        self._testme_std([                'resources/nginx-env.j2', 'resources/data.env'])
         # Format
         self._testme_std(['--format=env', 'resources/nginx-env.j2', 'resources/data.env'])
+        self._testme_std([                'resources/nginx-env.j2', 'resources/data.env'])
         # Stdin
-        self._testme_std(['--format=env', 'resources/nginx-env.j2'], stdin=open('resources/data.env'))
+        self._testme_std(['--format=env', 'resources/nginx-env.j2', '-'], stdin=open('resources/data.env'))
+        self._testme_std([                'resources/nginx-env.j2', '-'], stdin=open('resources/data.env'))
 
         # Environment!
+        # In this case, it's not explicitly provided, but implicitly gotten from the environment
         env = dict(NGINX_HOSTNAME='localhost', NGINX_WEBROOT='/var/www/project', NGINX_LOGS='/var/log/nginx/')
         self._testme_std(['--format=env', 'resources/nginx-env.j2'], env=env)
-        self._testme_std(['--format=env', 'resources/nginx-env.j2'], env=env)
+        self._testme_std([                'resources/nginx-env.j2'], env=env)
 
     def test_import_env(self):
         # Import environment into a variable
diff --git a/tests/resources/data-empty.env b/tests/resources/data-empty.env
new file mode 100644
index 0000000..e69de29