diff --git a/src/tracealign/io/sefaria.py b/src/tracealign/io/sefaria.py
index 8443e03..8163df8 100644
--- a/src/tracealign/io/sefaria.py
+++ b/src/tracealign/io/sefaria.py
@@ -52,11 +52,16 @@ def _fetch_json(url: str) -> dict:
 
 
 def _build_url(ref: str, version: str | None) -> str:
-    """Build the v3 API URL for a reference, optionally pinning a version."""
+    """Build the v3 API URL for a reference, optionally pinning a version.
+
+    The Sefaria v3 API accepts ``version=<language>|<versionTitle>``. The
+    language token must be the full name (``hebrew``), not the ISO code
+    (``he``); the API silently returns an empty versions array otherwise.
+    """
     encoded_ref = quote(ref.replace(" ", "_"), safe="._")
     url = f"{SEFARIA_API_BASE}/{encoded_ref}"
     if version is not None:
-        url += f"?version=he|{quote(version, safe='')}"
+        url += f"?version=hebrew|{quote(version, safe='')}"
     return url
 
 
diff --git a/tests/io/test_sefaria_fetch.py b/tests/io/test_sefaria_fetch.py
index e845bd9..e3586dc 100644
--- a/tests/io/test_sefaria_fetch.py
+++ b/tests/io/test_sefaria_fetch.py
@@ -30,3 +30,20 @@ def test_fetch_json_uses_fixture():
         result = sefaria._fetch_json("https://example.invalid/")
     assert result["ref"] == "Pirkei Avot 1"
     assert result["versions"][0]["versionTitle"].startswith("William Davidson")
+
+
+def test_build_url_uses_hebrew_language_token():
+    """Pin: Sefaria API requires 'hebrew|' not 'he|' for the version language."""
+    url = sefaria._build_url("Pirkei Avot 1", version="Vilna Edition")
+    assert "version=hebrew%7CVilna" in url or "version=hebrew|Vilna" in url
+    # Negative: the ISO-code form must not slip back in
+    assert "version=he|" not in url
+    assert "version=he%7C" not in url
+
+
+def test_build_url_without_version_has_no_query():
+    url = sefaria._build_url("Pirkei Avot 1", version=None)
+    assert "?" not in url
+    assert url.startswith("https://www.sefaria.org/api/v3/texts/")
+    # The reference must be URL-safe: no raw whitespace ends up in the URL.
+    assert " " not in url