diff --git a/rockcraft/pebble.py b/rockcraft/pebble.py
index f62e33ca3..87ff9cc67 100644
--- a/rockcraft/pebble.py
+++ b/rockcraft/pebble.py
@@ -194,16 +194,18 @@ def _get_check_tag(check: Mapping[str, Any] | _BaseCheck) -> str:
         return "tcp"
     if isinstance(check, ExecCheck):
         return "exec"
+    if not isinstance(check, Mapping):
+        raise CraftValidationError(f"Unknown check type for {check!r}.")
 
     tags = ("http", "tcp", "exec")
-    check_types = check.keys() & tags
+    check_types = [tag for tag in tags if tag in check]
     match len(check_types):
         case 0:
             raise CraftValidationError(
                 f"Must specify exactly one of {', '.join(tags)} for each check."
             )
         case 1:
-            return check_types.pop()
+            return check_types[0]
         case _:
             raise CraftValidationError(
                 f"Multiple check types specified ({', '.join(sorted(check_types))}). "
diff --git a/tests/unit/test_project.py b/tests/unit/test_project.py
index 101a3a82f..fe6c4f151 100644
--- a/tests/unit/test_project.py
+++ b/tests/unit/test_project.py
@@ -19,7 +19,7 @@
 import subprocess
 import warnings
 from pathlib import Path
-from typing import cast
+from typing import Any, cast
 
 import pydantic
 import pytest
@@ -884,7 +884,8 @@ def test_project_marshal_exec_check_emits_no_pydantic_warning():
         warnings.simplefilter("always")
         dumped = project.marshal()
 
-    assert dumped["checks"]["online"]["exec"]["command"] == "/bin/healthcheck"
+    checks = cast(dict[str, Any], dumped["checks"])
+    assert checks["online"]["exec"]["command"] == "/bin/healthcheck"
     assert not any(
         "PydanticSerializationUnexpectedValue" in str(w.message) for w in caught
     )