diff --git a/aws_project_costs/schema.py b/aws_project_costs/schema.py
index cd509bb..59616e8 100644
--- a/aws_project_costs/schema.py
+++ b/aws_project_costs/schema.py
@@ -31,7 +31,7 @@ def validate(projects, raise_on_error=False):
                 )
 
     for group in acc.get("project-groups", []):
-        if set(p["costshare"] for p in group) == {0}:
+        if set(p["costshare"] for p in projects["project-groups"][group]) == {0}:
             errors.append(f"All projects in project-group {group} have costshare=0")
 
     if raise_on_error and errors:
diff --git a/tests/test_project_costs.py b/tests/test_project_costs.py
index ab1caa7..649869f 100644
--- a/tests/test_project_costs.py
+++ b/tests/test_project_costs.py
@@ -4,19 +4,10 @@
 import yaml
 
 from aws_project_costs.project_costs import analyse_costs_csv
-from aws_project_costs.schema import validate
 
 EXAMPLE_DIR = Path(__file__).parent / ".." / "example"
 
 
-def test_schema_validate():
-    config_yaml = EXAMPLE_DIR / "projects.yaml"
-    with (config_yaml).open() as f:
-        cfg = yaml.safe_load(f)
-    errors = validate(cfg, raise_on_error=False)
-    assert len(errors) == 0
-
-
 def test_analyse_costs_csv(tmp_path):
     config_yaml = EXAMPLE_DIR / "projects.yaml"
     input_csv = EXAMPLE_DIR / "2024-01-01_2024-02-01.csv"
diff --git a/tests/test_schema.py b/tests/test_schema.py
new file mode 100644
index 0000000..f328962
--- /dev/null
+++ b/tests/test_schema.py
@@ -0,0 +1,72 @@
+from pathlib import Path
+
+import pytest
+import yaml
+
+from aws_project_costs.schema import validate
+
+EXAMPLE_DIR = Path(__file__).parent / ".." / "example"
+
+
+def test_schema_validate_example():
+    config_yaml = EXAMPLE_DIR / "projects.yaml"
+    with (config_yaml).open() as f:
+        cfg = yaml.safe_load(f)
+    errors = validate(cfg, raise_on_error=False)
+    assert len(errors) == 0
+
+
+@pytest.mark.parametrize("raise_on_error", [True, False])
+def test_schema_validate_errors(raise_on_error):
+    config_yaml = """
+proj-tag-names:
+  project-001: Project A
+
+accounts:
+  - name: aws-auth
+    billing-type: shared
+    project-tagname: Proj
+    project-groups:
+      - tre
+      - non-existent
+
+  - name: aws-AUTH
+    billing-type: project-specific
+    project: x
+
+  - name: invalid-costshare
+    billing-type: shared
+    project-groups:
+      - invalid
+
+project-groups:
+  tre:
+    - name: Project A
+      costshare: 1
+    - name: Project B
+      costshare: 0
+  web:
+    - name: Project A
+      costshare: 2
+    - name: Project C
+      costshare: 1
+  invalid:
+    - name: Project D
+      costshare: 0
+"""
+    cfg = yaml.safe_load(config_yaml)
+
+    if raise_on_error:
+        with pytest.raises(ValueError) as excinfo:
+            validate(cfg, raise_on_error=raise_on_error)
+        errors = excinfo.value.args[0]
+    else:
+        errors = validate(cfg, raise_on_error=raise_on_error)
+
+    assert len(errors) == 3
+    expected_errors = [
+        "project-group non-existent in account aws-auth does not exist!",
+        "Multiple entries for account name=aws-auth",
+        "All projects in project-group invalid have costshare=0",
+    ]
+    assert errors == expected_errors