diff --git a/pyproject.toml b/pyproject.toml
index 545bd287..df04fca7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -69,7 +69,7 @@ dependencies = [
     "pystow>=0.6.0",
     "bioversions>=0.5.535",
     "bioregistry>=0.11.23",
-    "bioontologies>=0.4.0",
+    "bioontologies>=0.5.2",
     "zenodo-client>=0.0.5",
     "class_resolver",
     "psycopg2-binary",
diff --git a/src/pyobo/obographs.py b/src/pyobo/obographs.py
index 3329efe9..c3c21ddf 100644
--- a/src/pyobo/obographs.py
+++ b/src/pyobo/obographs.py
@@ -69,9 +69,7 @@ def _rewire(r: curies.Reference | Referenced) -> curies.Reference:
 
 
 def _get_class_node(term: Term) -> Node:
-    if term.provenance and not term.definition:
-        logger.warning("[%s] unhandled when provenance but no definition", term.curie)
-    elif term.definition:
+    if term.provenance or term.definition:
         definition = Definition.from_parsed(
             value=term.definition, references=[_rewire(p) for p in term.provenance or []]
         )
diff --git a/src/pyobo/reader.py b/src/pyobo/reader.py
index d8390f76..65f3331f 100644
--- a/src/pyobo/reader.py
+++ b/src/pyobo/reader.py
@@ -160,8 +160,7 @@ def from_obonet(graph: nx.MultiDiGraph, *, strict: bool = True) -> Obo:
         n_xrefs += len(xrefs)
 
         definition, definition_references = get_definition(data, node=reference)
-        if definition_references:
-            provenance.extend(definition_references)
+        provenance.extend(definition_references)
 
         alt_ids = list(iterate_node_alt_ids(data, strict=strict))
         n_alt_ids += len(alt_ids)
@@ -342,11 +341,11 @@ def iterate_graph_typedefs(
         yield TypeDef(reference=reference, xrefs=xrefs)
 
 
-def get_definition(data, *, node: Reference) -> tuple[None, None] | tuple[str, list[Reference]]:
+def get_definition(data, *, node: Reference) -> tuple[None | str, list[Reference]]:
     """Extract the definition from the data."""
     definition = data.get("def")  # it's allowed not to have a definition
     if not definition:
-        return None, None
+        return None, []
     return _extract_definition(definition, node=node)
 
 
@@ -355,17 +354,17 @@ def _extract_definition(
     *,
     node: Reference,
     strict: bool = False,
-) -> tuple[None, None] | tuple[str, list[Reference]]:
+) -> tuple[None | str, list[Reference]]:
     """Extract the definitions."""
     if not s.startswith('"'):
         logger.warning(f"[{node.curie}] definition does not start with a quote")
-        return None, None
+        return None, []
 
     try:
         definition, rest = _quote_split(s)
     except ValueError as e:
         logger.warning("[%s] failed to parse definition quotes: %s", node.curie, str(e))
-        return None, None
+        return None, []
 
     if not rest.startswith("[") or not rest.endswith("]"):
         logger.warning(
@@ -374,7 +373,7 @@ def _extract_definition(
         provenance = []
     else:
         provenance = _parse_trailing_ref_list(rest, strict=strict, node=node)
-    return definition, provenance
+    return definition or None, provenance
 
 
 def get_first_nonescaped_quote(s: str) -> int | None:
@@ -389,7 +388,9 @@ def get_first_nonescaped_quote(s: str) -> int | None:
 
 
 def _quote_split(s: str) -> tuple[str, str]:
-    s = s.lstrip('"')
+    if not s.startswith('"'):
+        raise ValueError(f"'{s}' does not start with a quote")
+    s = s.removeprefix('"')
     i = get_first_nonescaped_quote(s)
     if i is None:
         raise ValueError(f"no closing quote found in `{s}`")
diff --git a/src/pyobo/struct/reference.py b/src/pyobo/struct/reference.py
index 46222c9d..0d82ee73 100644
--- a/src/pyobo/struct/reference.py
+++ b/src/pyobo/struct/reference.py
@@ -218,3 +218,8 @@ def reference_escape(predicate: Reference | Referenced, *, ontology_prefix: str)
         return predicate.identifier.removeprefix(f"{ontology_prefix}#")
     else:
         return predicate.preferred_curie
+
+
+def comma_separate_references(references: list[Reference]) -> str:
+    """Map a list to strings and make comma separated."""
+    return ", ".join(r.preferred_curie for r in references)
diff --git a/src/pyobo/struct/struct.py b/src/pyobo/struct/struct.py
index 34230b0c..39114832 100644
--- a/src/pyobo/struct/struct.py
+++ b/src/pyobo/struct/struct.py
@@ -25,7 +25,13 @@
 from tqdm.auto import tqdm
 from typing_extensions import Self
 
-from .reference import Reference, Referenced, default_reference, reference_escape
+from .reference import (
+    Reference,
+    Referenced,
+    comma_separate_references,
+    default_reference,
+    reference_escape,
+)
 from .typedef import (
     TypeDef,
     comment,
@@ -40,7 +46,7 @@
     see_also,
     term_replaced_by,
 )
-from .utils import comma_separate, obo_escape_slim
+from .utils import obo_escape_slim
 from ..api.utils import get_version
 from ..constants import (
     DATE_FORMAT,
@@ -109,7 +115,7 @@ def _fp(self, ontology_prefix: str) -> str:
         x = f'"{self._escape(self.name)}" {self.specificity}'
         if self.type and self.type.pair != DEFAULT_SYNONYM_TYPE.pair:
             x = f"{x} {reference_escape(self.type, ontology_prefix=ontology_prefix)}"
-        return f"{x} [{comma_separate(self.provenance)}]"
+        return f"{x} [{comma_separate_references(self.provenance)}]"
 
     @staticmethod
     def _escape(s: str) -> str:
@@ -454,9 +460,8 @@ def annotate_boolean(self, prop: ReferenceHint, value: bool) -> Self:
         )
 
     def _definition_fp(self) -> str:
-        if self.definition is None:
-            raise AssertionError
-        return f'"{obo_escape_slim(self.definition)}" [{comma_separate(self.provenance)}]'
+        definition = obo_escape_slim(self.definition) if self.definition else ""
+        return f'"{definition}" [{comma_separate_references(self.provenance)}]'
 
     def iterate_relations(self) -> Iterable[tuple[Reference, Reference]]:
         """Iterate over pairs of typedefs and targets."""
@@ -498,11 +503,8 @@ def iterate_obo_lines(
 
         xrefs = list(self.xrefs)
 
-        if self.definition:
+        if self.definition or self.provenance:
             yield f"def: {self._definition_fp()}"
-        elif self.provenance:
-            # if no definition, just stick on xrefs
-            xrefs.extend(self.provenance)
 
         for alt in sorted(self.alt_ids):
             yield f"alt_id: {alt}"  # __str__ bakes in the ! name
@@ -1144,7 +1146,7 @@ def to_obonet(self: Obo, *, use_tqdm: bool = False) -> nx.MultiDiGraph:
             d = {
                 "id": term.curie,
                 "name": term.name,
-                "def": term.definition and term._definition_fp(),
+                "def": (term.definition or term.provenance) and term._definition_fp(),
                 "xref": [xref.curie for xref in term.xrefs],
                 "is_a": parents,
                 "relationship": relations,
diff --git a/src/pyobo/struct/utils.py b/src/pyobo/struct/utils.py
index 344fe514..d30dfb7b 100644
--- a/src/pyobo/struct/utils.py
+++ b/src/pyobo/struct/utils.py
@@ -5,7 +5,6 @@
 __all__ = [
     "OBO_ESCAPE",
     "OBO_ESCAPE_SLIM",
-    "comma_separate",
     "obo_escape",
     "obo_escape_slim",
 ]
@@ -24,8 +23,3 @@ def obo_escape_slim(string: str) -> str:
     rv = "".join(OBO_ESCAPE_SLIM.get(character, character) for character in string)
     rv = rv.replace("\n", "\\n")
     return rv
-
-
-def comma_separate(elements) -> str:
-    """Map a list to strings and make comma separated."""
-    return ", ".join(map(str, elements))
diff --git a/tests/test_reader.py b/tests/test_reader.py
index 8537e97c..01a9b767 100644
--- a/tests/test_reader.py
+++ b/tests/test_reader.py
@@ -466,6 +466,20 @@ def test_definition_with_provenance(self) -> None:
         self.assertEqual(1, len(term.provenance))
         self.assertEqual(CHARLIE, term.provenance[0])
 
+    def test_provenance_no_definition(self) -> None:
+        """Test parsing a term with provenance but no definition."""
+        ontology = _read(f"""\
+            ontology: chebi
+
+            [Term]
+            id: CHEBI:1234
+            def: "" [{CHARLIE.curie}]
+        """)
+        term = self.get_only_term(ontology)
+        self.assertIsNone(term.definition)
+        self.assertEqual(1, len(term.provenance))
+        self.assertEqual(CHARLIE, term.provenance[0])
+
     def test_synonym_minimal(self) -> None:
         """Test parsing a synonym just the text."""
         ontology = _read("""\
diff --git a/tests/test_struct.py b/tests/test_struct.py
index 04f17820..233dd10e 100644
--- a/tests/test_struct.py
+++ b/tests/test_struct.py
@@ -425,7 +425,7 @@ def test_provenance_no_definition(self) -> None:
             [Term]
             id: GO:0050069
             name: lysine dehydrogenase activity
-            xref: orcid:0000-0003-4423-4370
+            def: "" [orcid:0000-0003-4423-4370]
             """,
             term.iterate_obo_lines(ontology_prefix="go", typedefs={}),
         )