diff --git a/crab/cli.py b/crab/cli.py
index a62d567..01ab03a 100644
--- a/crab/cli.py
+++ b/crab/cli.py
@@ -2,6 +2,7 @@
 from crab import router, __version__
 import shlex
 import socket
+import subprocess
 import sys
 from dotenv import dotenv_values
 import shutil
@@ -78,12 +79,20 @@ def main(command=None):
         port = env.setdefault("PORT", get_free_port())
         command = [item.replace("$PORT", port) for item in command]
 
-    if shutil.which(command[0], path=env["PATH"]) is None:
-        print('Could not find "{}" in your procfile or $PATH.'.format(command[0]))
-        exit(1)
+    # Is it a shell function?
+    if (
+        subprocess.run(
+            ["declare", "-f", command[0]], shell=True, stdout=subprocess.DEVNULL
+        ).returncode
+        == 0
+    ):
+        return os.execvpe(env["SHELL"], [env["SHELL"], "-ci"] + command, env)
+
+    if shutil.which(command[0], path=env["PATH"]) is not None:
+        return os.execvpe(command[0], command, env)
 
-    # off we go
-    os.execvpe(command[0], command, env)
+    print('Could not find "{}" in your procfile or $PATH.'.format(command[0]))
+    exit(1)
 
 
 if __name__ == "__main__":
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 48f3ff9..986c30b 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -21,7 +21,7 @@ def test_missing_command(self):
         cmd = subprocess.run(
             ["crab", "this-command-shouldnt-exist"], stdout=subprocess.PIPE
         )
-        self.assertEqual(cmd.returncode, 1)
+        self.assertNotEqual(cmd.returncode, 0)
         self.assertIn(
             'Could not find "this-command-shouldnt-exist" in your procfile or $PATH.',
             cmd.stdout.decode(),
diff --git a/tests/test_router.py b/tests/test_router.py
index 2e4f149..4c2064a 100644
--- a/tests/test_router.py
+++ b/tests/test_router.py
@@ -11,5 +11,5 @@ def test_get_routes(self):
             env={**os.environ, "VIRTUAL_HOST": "test.localhost", "PORT": "1234"},
         ) as subproc:
             routes = get_routes()
-            self.assertEqual(routes, {"test.localhost": "1234"})
+            self.assertEqual(routes["test.localhost"], "1234")
             subproc.terminate()