diff --git a/nested_diff/cli.py b/nested_diff/cli.py
index 9f7ca0d..13cc65a 100644
--- a/nested_diff/cli.py
+++ b/nested_diff/cli.py
@@ -157,8 +157,12 @@ def get_dumper(fmt, **kwargs):
 
         raise RuntimeError(f'Unsupported output format: {fmt}')
 
-    @staticmethod
-    def guess_fmt(fp, default, ignore_fps=(sys.stdin, sys.stdout, sys.stderr)):
+    def guess_fmt(
+        self,
+        fp,
+        default,
+        ignore_fps=(sys.stdin, sys.stdout, sys.stderr),
+    ):
         """Guess format of a file object according it's extension."""
         if fp in ignore_fps:
             return default
@@ -168,7 +172,7 @@ def guess_fmt(fp, default, ignore_fps=(sys.stdin, sys.stdout, sys.stderr)):
         if fmt == 'yml':
             fmt = 'yaml'
 
-        return fmt if fmt else default
+        return fmt if fmt in self.supported_ifmts else default
 
     @staticmethod
     def get_loader(fmt, **kwargs):
@@ -209,7 +213,7 @@ def load(self, file_):
 
         """
         if self.args.ifmt == 'auto':
-            fmt = self.guess_fmt(file_, 'json')
+            fmt = self.guess_fmt(file_, 'plaintext')
         else:
             fmt = self.args.ifmt
 
diff --git a/tests/cli/shared.a.txt b/tests/cli/shared.a.txt
new file mode 100644
index 0000000..4cb29ea
--- /dev/null
+++ b/tests/cli/shared.a.txt
@@ -0,0 +1,3 @@
+one
+two
+three
diff --git a/tests/cli/shared.b.txt b/tests/cli/shared.b.txt
new file mode 100644
index 0000000..814f4a4
--- /dev/null
+++ b/tests/cli/shared.b.txt
@@ -0,0 +1,2 @@
+one
+two
diff --git a/tests/cli/test_cli.py b/tests/cli/test_cli.py
index ef0c37b..e21fc9a 100644
--- a/tests/cli/test_cli.py
+++ b/tests/cli/test_cli.py
@@ -72,18 +72,22 @@ def test_excepthook_raise_system_exit_127():
     assert e.value.code == 127
 
 
-def test_guess_fmt_aliases():
+def test_guess_fmt():
     class FakeFP:
         name = None
 
-    aliases = {
+    exts = {
+        'ini': 'ini',
+        'json': 'json',
+        'py': 'default',
+        'txt': 'default',
         'yml': 'yaml',
     }
     fake_fp = FakeFP()
 
-    for ext in sorted(aliases):
+    for ext, fmt in sorted(exts.items()):
         fake_fp.name = f'filename.{ext}'
-        assert cli.App(args=()).guess_fmt(fake_fp, 'default') == aliases[ext]
+        assert cli.App(args=()).guess_fmt(fake_fp, 'default') == fmt
 
 
 def test_guess_fmt_ignore_fp_defaults():
diff --git a/tests/cli/test_diff_tool.py b/tests/cli/test_diff_tool.py
index aa4531f..858263a 100644
--- a/tests/cli/test_diff_tool.py
+++ b/tests/cli/test_diff_tool.py
@@ -284,6 +284,21 @@ def test_html_ofmt_wrappings(capsys, expected, rpath):
     assert captured.out == expected
 
 
+def test_fallback_ifmt(capsys, expected, rpath):
+    exit_code = nested_diff.diff_tool.App(
+        args=(
+            rpath('shared.a.txt'),
+            rpath('shared.b.txt'),
+        ),
+    ).run()
+
+    captured = capsys.readouterr()
+    assert captured.err == ''
+    assert exit_code == 1
+
+    assert captured.out == expected
+
+
 def test_plaintext_ifmt(capsys, expected, rpath):
     exit_code = nested_diff.diff_tool.App(
         args=(
diff --git a/tests/cli/test_diff_tool.test_fallback_ifmt.exp b/tests/cli/test_diff_tool.test_fallback_ifmt.exp
new file mode 100644
index 0000000..bc10e83
--- /dev/null
+++ b/tests/cli/test_diff_tool.test_fallback_ifmt.exp
@@ -0,0 +1,6 @@
+# <str>
+  @@ -1,4 +1,3 @@
+  one
+  two
+- three
+