diff --git a/botocove/cove_host_account.py b/botocove/cove_host_account.py
index 403f63a..6e1b664 100644
--- a/botocove/cove_host_account.py
+++ b/botocove/cove_host_account.py
@@ -59,7 +59,7 @@ def __init__(
         self.host_account_partition = caller_id["Arn"].split(":")[1]
 
         if regions is None:
-            self.target_regions = [None]
+            self.target_regions = [assuming_session.region_name]
         else:
             self.target_regions = regions
 
diff --git a/pyproject.toml b/pyproject.toml
index 81f5d42..2fcfbff 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -32,7 +32,8 @@ flake8-print = ">=5.0.0"
 mypy = ">=1.5.1"
 pre-commit = ">=3.4.0"
 flakeheaven = ">=3.3.0"
-moto = {extras = ["organizations", "sts"], version = ">=4.2.5"}
+moto = {extras = ["organizations", "sts", "ec2"], version = ">=4.2.5"}
+boto3-stubs = {extras = ["ec2"], version = "*"}
 pytest-randomly = ">=3.15.0"
 
 # These are the last versions compatible with flake8 4. flakeheaven 3.3.0 is
diff --git a/tests/conftest.py b/tests/conftest.py
index 5d6326e..90453e3 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -24,6 +24,16 @@ def _clean_env(monkeypatch: MonkeyPatch) -> None:
         monkeypatch.setenv(env_var, "broken_not_real_profile")
 
 
+@pytest.fixture(autouse=True)
+def _default_region(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.setenv("AWS_DEFAULT_REGION", "eu-west-1")
+
+
+@pytest.fixture()
+def _no_default_region(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.delenv("AWS_DEFAULT_REGION")
+
+
 @pytest.fixture()
 def mock_session() -> Iterator[Session]:
     """Returns a session with mock AWS services."""
diff --git a/tests/test_assuming_session.py b/tests/test_assuming_session.py
new file mode 100644
index 0000000..4083a7a
--- /dev/null
+++ b/tests/test_assuming_session.py
@@ -0,0 +1,53 @@
+import pytest
+from boto3.session import Session
+from botocore.exceptions import NoRegionError
+from moto import mock_ec2
+
+from botocove import cove
+
+# Query the region with different configurations of assuming session and
+# environment variables. To make the assertions easier all `cove` calls set
+# `raise_exception`. If set the assuming session region and the default region
+# are always distinct to be able to assert the source of the query result.
+
+def _query_region(session: Session) -> str:
+    with mock_ec2():
+        response = session.client("ec2").describe_availability_zones()
+        return response["AvailabilityZones"][0]["RegionName"]
+
+
+@pytest.fixture(autouse=True)
+def _org_with_one_member(mock_session: Session) -> None:
+    org_client = mock_session.client("organizations")
+    org_client.create_organization(FeatureSet="ALL")
+    org_client.create_account(Email="account1@aws.com", AccountName="Account 1")
+
+
+@pytest.mark.usefixtures("_no_default_region")
+def test_when_no_assuming_session_and_no_default_region_then_cove_raises_error() -> None:  # noqa: 501
+    with pytest.raises(NoRegionError, match=r"^You must specify a region\.$"):
+        cove(_query_region, raise_exception=True)()
+
+
+def test_when_no_assuming_session_then_cove_uses_default_region() -> None:
+    output = cove(_query_region, raise_exception=True)()
+    assert output["Results"][0]["Result"] == "eu-west-1"
+
+
+@pytest.mark.usefixtures("_no_default_region")
+def test_when_no_default_region_then_cove_uses_assuming_session_region() -> None:
+    output = cove(
+        _query_region,
+        assuming_session=Session(region_name="eu-central-1"),
+        raise_exception=True,
+    )()
+    assert output["Results"][0]["Result"] == "eu-central-1"
+
+
+def test_cove_prefers_assuming_session_region() -> None:
+    output = cove(
+        _query_region,
+        assuming_session=Session(region_name="eu-central-1"),
+        raise_exception=True,
+    )()
+    assert output["Results"][0]["Result"] == "eu-central-1"
diff --git a/tests/test_decorator.py b/tests/test_decorator.py
index 650f2ec..ce1bf08 100644
--- a/tests/test_decorator.py
+++ b/tests/test_decorator.py
@@ -86,6 +86,7 @@ def simple_func(session: CoveSession, arg1: int, arg2: int, arg3: int) -> int:
             "Result": 6,
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
+            "Region": "eu-west-1",
         }
     ]
 
diff --git a/tests/test_decorator_no_args.py b/tests/test_decorator_no_args.py
index 5fe4f89..71985f9 100644
--- a/tests/test_decorator_no_args.py
+++ b/tests/test_decorator_no_args.py
@@ -35,6 +35,7 @@ def simple_func(session: CoveSession) -> str:
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
             "Result": "hello",
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output["Results"] == expected
@@ -58,6 +59,7 @@ def simple_func(session: CoveSession, output: str) -> str:
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
             "Result": "blue",
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output["Results"] == expected
@@ -86,6 +88,7 @@ def simple_func(
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
             "Result": ("blue", "circle", "11:11"),
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output == expected
diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
index 0a66f97..7143579 100644
--- a/tests/test_exceptions.py
+++ b/tests/test_exceptions.py
@@ -56,6 +56,7 @@ def simple_func(session: CoveSession) -> None:
         "Status": "ACTIVE",
         "RoleSessionName": "OrganizationAccountAccessRole",
         "ExceptionDetails": repr(Exception("oh no")),
+        "Region": "eu-west-1",
     }
 
     # Compare repr of exceptions
diff --git a/tests/test_regions.py b/tests/test_regions.py
index 02d7719..40fd4eb 100644
--- a/tests/test_regions.py
+++ b/tests/test_regions.py
@@ -5,7 +5,7 @@
 from tests.moto_mock_org.moto_models import SmallOrg
 
 
-def test_when_region_is_unspecified_then_result_has_no_region_key(
+def test_when_region_is_unspecified_then_result_has_default_region_key(
     mock_small_org: SmallOrg,
 ) -> None:
     @cove()
@@ -15,7 +15,7 @@ def do_nothing(session: Session) -> None:
     output = do_nothing()
     assert output["Results"]
     for result in output["Results"]:
-        assert "Region" not in result
+        assert result["Region"] == "eu-west-1"
 
 
 def test_when_region_is_unspecified_then_output_has_one_result_per_account(
@@ -26,20 +26,16 @@ def do_nothing(session: Session) -> None:
         pass
 
     output = do_nothing()
-    print(output["Results"])
-    print(len(output["Results"]))
-    print(_count_member_accounts(mock_session))
-    print(mock_small_org.all_accounts)
     assert len(output["Results"]) == _count_member_accounts(mock_session)
 
 
 def test_when_region_is_str_then_raises_type_error(mock_small_org: SmallOrg) -> None:
-    @cove(regions="eu-west-1")  # type: ignore[arg-type]
+    @cove(regions="eu-central-1")  # type: ignore[arg-type]
     def do_nothing() -> None:
         pass
 
     with pytest.raises(
-        TypeError, match=r"regions must be a list of str\. Got str 'eu-west-1'\."
+        TypeError, match=r"regions must be a list of str\. Got str 'eu-central-1'\."
     ):
         do_nothing()
 
@@ -58,20 +54,20 @@ def do_nothing() -> None:
 def test_when_any_region_is_passed_then_result_has_region_key(
     mock_small_org: SmallOrg,
 ) -> None:
-    @cove(regions=["eu-west-1"])
+    @cove(regions=["eu-central-1"])
     def do_nothing(session: Session) -> None:
         pass
 
     output = do_nothing()
     assert output["Results"]
     for result in output["Results"]:
-        assert result["Region"] == "eu-west-1"
+        assert result["Region"] == "eu-central-1"
 
 
 def test_when_two_regions_are_passed_then_output_has_one_result_per_account_per_region(
     mock_session: Session, mock_small_org: SmallOrg
 ) -> None:
-    @cove(regions=["eu-west-1", "us-east-1"])
+    @cove(regions=["eu-central-1", "us-east-1"])
     def do_nothing(session: Session) -> None:
         pass
 
@@ -79,7 +75,7 @@ def do_nothing(session: Session) -> None:
 
     number_of_member_accounts = _count_member_accounts(mock_session)
 
-    for region in ["eu-west-1", "us-east-1"]:
+    for region in ["eu-central-1", "us-east-1"]:
         number_of_results_per_region = sum(
             1 for result in output["Results"] if result["Region"] == region
         )
diff --git a/tests/test_session.py b/tests/test_session.py
index 4d609aa..2626637 100644
--- a/tests/test_session.py
+++ b/tests/test_session.py
@@ -37,6 +37,7 @@ def simple_func(session: CoveSession, a_string: str) -> str:
             "Result": "test-string",
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output["Results"] == expected
@@ -66,6 +67,7 @@ def simple_func(session: CoveSession, a_string: str) -> str:
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
             "Policy": session_policy,
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output["Results"] == expected
@@ -97,6 +99,7 @@ def simple_func(session: CoveSession, a_string: str) -> str:
             "RoleName": "OrganizationAccountAccessRole",
             "RoleSessionName": "OrganizationAccountAccessRole",
             "PolicyArns": session_policy_arns,
+            "Region": "eu-west-1",
         }
     ]
     assert cove_output["Results"] == expected