diff --git a/CHANGES.md b/CHANGES.md
index 4680637..055d833 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -9,6 +9,10 @@
 * Performance improvements and simplification to habitat processing.
 * Store more analysis data from model validation.
 
+### Fixed
+
+* Fixed a bug in collate data where it would fail to process any files.
+
 ## v1.0.1 (19/10/2025)
 
 ### Fixed
diff --git a/aoh/validation/collate_data.py b/aoh/validation/collate_data.py
index fcce4e4..e55ac00 100644
--- a/aoh/validation/collate_data.py
+++ b/aoh/validation/collate_data.py
@@ -29,9 +29,11 @@ def collate_data(
     aoh_results: Path,
     output_path: Path,
 ) -> None:
-    manifests = aoh_results.glob("**/*.json")
-    if len(list(manifests)) == 0:
-        sys.exit(f"Found no manifests in {aoh_results}")
+    # Casting to a list here is a bit wasteful, but I think getting a sense
+    # that there is no files early leads to better error reporting.
+    manifests = list(aoh_results.glob("**/*.json"))
+    if len(manifests) == 0:
+        raise FileNotFoundError(f"Found no manifests in {aoh_results}")
 
     os.makedirs(output_path.parent, exist_ok=True)
 
@@ -73,10 +75,13 @@ def main() -> None:
     )
     args = parser.parse_args()
 
-    collate_data(
-        args.aohs_path,
-        args.output_path,
-    )
+    try:
+        collate_data(
+            args.aohs_path,
+            args.output_path,
+        )
+    except FileNotFoundError:
+        sys.exit("Failed to find data")
 
 if __name__ == "__main__":
     main()
diff --git a/tests/test_collate_data.py b/tests/test_collate_data.py
new file mode 100644
index 0000000..c092d72
--- /dev/null
+++ b/tests/test_collate_data.py
@@ -0,0 +1,23 @@
+import tempfile
+from pathlib import Path
+
+import pytest
+import pandas as pd
+
+from aoh.validation.collate_data import collate_data
+
+def test_simple_positive_path() -> None:
+    with tempfile.TemporaryDirectory() as tmpdir:
+        src_data_path = Path("tests/testdata/collate_data")
+        input_file_count = len(list(src_data_path.glob("*.json")))
+
+        output_path = Path(tmpdir) / "res.csv"
+        collate_data(src_data_path, output_path)
+
+        results = pd.read_csv(output_path)
+        assert len(results) == input_file_count
+
+def test_fails_on_empty_dir() -> None:
+    with tempfile.TemporaryDirectory() as tmpdir:
+        with pytest.raises(FileNotFoundError):
+            collate_data(Path(tmpdir), Path(tmpdir) / "output.csv")
diff --git a/tests/testdata/collate_data/1.json b/tests/testdata/collate_data/1.json
new file mode 100644
index 0000000..76eeda9
--- /dev/null
+++ b/tests/testdata/collate_data/1.json
@@ -0,0 +1,18 @@
+{
+  "id_no": "1",
+  "assessment_id": "111",
+  "assessment_year": "2010",
+  "season": "RESIDENT",
+  "elevation_lower": "0",
+  "elevation_upper": "150",
+  "full_habitat_code": "3.5|14.3|4.4|3.7|3.6|15.8|4.7|14.6|4.5|14.2|3.4|4.6|14.5|14.1|15.7|15.9|14.4",
+  "scientific_name": "One une en",
+  "family_name": "NUMBER",
+  "class_name": "MAMMALIA",
+  "category": "LC",
+  "range_total": 5377499278409.75,
+  "hab_total": 3142935478203.522,
+  "dem_total": 5118755494259.25,
+  "aoh_total": 3026810920143.975,
+  "prevalence": 0.5628658905258045
+}
diff --git a/tests/testdata/collate_data/2.json b/tests/testdata/collate_data/2.json
new file mode 100644
index 0000000..c93c1d2
--- /dev/null
+++ b/tests/testdata/collate_data/2.json
@@ -0,0 +1,18 @@
+{
+  "id_no": "2",
+  "assessment_id": "222",
+  "assessment_year": "2020",
+  "season": "RESIDENT",
+  "elevation_lower": "20",
+  "elevation_upper": "300",
+  "full_habitat_code": "3.4|14.3|4.4|1.4|6|3.8|1.3|3.2",
+  "scientific_name": "Two deux två",
+  "family_name": "NUMBER",
+  "class_name": "MAMMALIA",
+  "category": "EN",
+  "range_total": 133914029408.875,
+  "hab_total": 88190380681.45854,
+  "dem_total": 129205940713.25,
+  "aoh_total": 86741613961.4514,
+  "prevalence": 0.6477410495699915
+}