diff --git a/envs/failing-env2.yaml b/envs/failing-env2.yaml
new file mode 100644
index 0000000..4aa11c8
--- /dev/null
+++ b/envs/failing-env2.yaml
@@ -0,0 +1,4 @@
+channels:
+  - conda-forge
+dependencies:
+  - package_no_version_not_in_exclude
diff --git a/minimum_versions.py b/minimum_versions.py
index f2bdaa2..559e694 100644
--- a/minimum_versions.py
+++ b/minimum_versions.py
@@ -153,6 +153,22 @@ def parse_environment(text):
     return specs, warnings
 
 
+def assert_spec_has_version(environments):
+    # packages in policy.exclude were already removed
+
+    for env, specs in environments.items():
+        for spec in specs:
+            if spec.version is None:
+
+                msg = (
+                    f"No minimum version found for '{spec.name}' in '{env}'. Either"
+                    " add a version or add to the list of excluded packages in the"
+                    " policy file."
+                )
+
+                raise ValueError(msg)
+
+
 def parse_policy(file):
     policy = yaml.safe_load(file)
     try:
@@ -344,6 +360,10 @@ def parse_date(string):
 @click.option("--today", type=parse_date, default=None)
 @click.option("--policy", "policy_file", type=click.File(mode="r"), required=True)
 def main(today, policy_file, environment_paths):
+    _main(today, policy_file, environment_paths)
+
+
+def _main(today, policy_file, environment_paths):
     console = Console()
 
     policy = parse_policy(policy_file)
@@ -360,6 +380,8 @@ def main(today, policy_file, environment_paths):
         for env, (specs, _) in parsed_environments.items()
     }
 
+    assert_spec_has_version(environments)
+
     all_packages = list(
         dict.fromkeys(spec.name for spec in concat(environments.values()))
     )
diff --git a/test_script.py b/test_script.py
index 22a62f9..577d0e3 100644
--- a/test_script.py
+++ b/test_script.py
@@ -1,9 +1,10 @@
 import datetime as dt
+import pathlib
 
 import pytest
 from rattler import Version
 
-from minimum_versions import Policy, Release, Spec
+from minimum_versions import Policy, Release, Spec, _main
 
 
 @pytest.mark.parametrize(
@@ -27,6 +28,22 @@ def test_spec_parse(text, expected_spec, expected_name, expected_warnings):
     assert actual_warnings == expected_warnings
 
 
+def test_error_missing_version_or_exclude():
+
+    msg = (
+        "No minimum version found for 'package_no_version_not_in_exclude' in"
+        " 'failing-env2'. Either add a version or add to the list of excluded packages"
+        " in the policy file."
+    )
+
+    with open("policy.yaml") as policy:
+
+        environment_paths = [pathlib.Path("envs/failing-env2.yaml")]
+
+        with pytest.raises(ValueError, match=msg):
+            _main(dt.date(2023, 12, 12), policy, environment_paths=environment_paths)
+
+
 @pytest.mark.parametrize(
     ["package_name", "policy", "today", "expected"],
     (