diff --git a/newsfragments/reject_non_list_security_opt.bugfix b/newsfragments/reject_non_list_security_opt.bugfix
new file mode 100644
index 00000000..86263e48
--- /dev/null
+++ b/newsfragments/reject_non_list_security_opt.bugfix
@@ -0,0 +1,2 @@
+Reject a non-list ``security_opt`` in service definitions instead of silently
+coercing a bare scalar into a one-element list.
diff --git a/podman_compose.py b/podman_compose.py
index fb8d31aa..8f578e62 100755
--- a/podman_compose.py
+++ b/podman_compose.py
@@ -2099,13 +2099,15 @@ def normalize_service(service: dict[str, Any], sub_dir: str = "") -> dict[str, A
     if "build" in service and "args" in service["build"]:
         if isinstance(build["args"], dict):
             build["args"] = norm_as_list(build["args"])
-    for key in ("env_file", "security_opt", "volumes"):
+    for key in ("env_file", "volumes"):
         if key not in service:
             continue
         if isinstance(service[key], str):
             service[key] = [service[key]]
     if "security_opt" in service:
         sec_ls = service["security_opt"]
+        if not is_list(sec_ls):
+            raise PodmanComposeError("ERROR: security_opt must be a list")
         for ix, item in enumerate(sec_ls):
             if item in ("seccomp:unconfined", "apparmor:unconfined"):
                 sec_ls[ix] = item.replace(":", "=")
diff --git a/tests/unit/test_normalize_service.py b/tests/unit/test_normalize_service.py
index 9907c84b..48aec98b 100644
--- a/tests/unit/test_normalize_service.py
+++ b/tests/unit/test_normalize_service.py
@@ -5,6 +5,7 @@
 
 from parameterized import parameterized
 
+from podman_compose import PodmanComposeError
 from podman_compose import normalize_service
 
 
@@ -117,3 +118,17 @@ def test_command_like(
             expected_service = {}
             expected_service[key] = expected
             self.assertEqual(normalize_service(input_service), expected_service)
+
+    @parameterized.expand([
+        ("label=disable",),
+        ({"label": "disable"},),
+    ])
+    def test_security_opt_must_be_a_list(self, value: Any) -> None:
+        with self.assertRaises(PodmanComposeError):
+            normalize_service({"security_opt": value})
+
+    def test_security_opt_list_is_normalized(self) -> None:
+        self.assertEqual(
+            normalize_service({"security_opt": ["seccomp:unconfined", "label=disable"]}),
+            {"security_opt": ["seccomp=unconfined", "label=disable"]},
+        )