diff --git a/masque/file/oasis.py b/masque/file/oasis.py
index 95a2039..b5d0cd8 100644
--- a/masque/file/oasis.py
+++ b/masque/file/oasis.py
@@ -562,7 +562,9 @@ def _shapes_to_elements(
                 xy = rint_cast(shape.offset + shape.vertices[0] + rep_offset)
                 deltas = rint_cast(numpy.diff(shape.vertices, axis=0))
                 half_width = rint_cast(shape.width / 2)
-                path_type = next(k for k, v in path_cap_map.items() if v == shape.cap)    # reverse lookup
+                path_type = next((k for k, v in path_cap_map.items() if v == shape.cap), None)    # reverse lookup
+                if path_type is None:
+                    raise PatternError(f'OASIS writer does not support path cap {shape.cap}')
                 extension_start = (path_type, shape.cap_extensions[0] if shape.cap_extensions is not None else None)
                 extension_end = (path_type, shape.cap_extensions[1] if shape.cap_extensions is not None else None)
                 path = fatrec.Path(
diff --git a/masque/test/test_oasis.py b/masque/test/test_oasis.py
index ad9bbf9..f549db7 100644
--- a/masque/test/test_oasis.py
+++ b/masque/test/test_oasis.py
@@ -1,9 +1,12 @@
+import io
 from pathlib import Path
 import pytest
 from numpy.testing import assert_equal
 
+from ..error import PatternError
 from ..pattern import Pattern
 from ..library import Library
+from ..shapes import Path as MPath
 
 
 def test_oasis_roundtrip(tmp_path: Path) -> None:
@@ -42,3 +45,16 @@ def test_oasis_properties_to_annotations_merges_repeated_keys() -> None:
     )
 
     assert annotations == {"k": [1, 2, 3]}
+
+
+def test_oasis_write_rejects_circle_path_caps() -> None:
+    pytest.importorskip("fatamorgana")
+    from ..file import oasis
+
+    lib = Library()
+    pat = Pattern()
+    pat.path((1, 0), vertices=[[0, 0], [10, 0]], width=2, cap=MPath.Cap.Circle)
+    lib["cell1"] = pat
+
+    with pytest.raises(PatternError, match="does not support path cap"):
+        oasis.write(lib, io.BytesIO(), units_per_micron=1000)