diff --git a/src/tenantq/search.py b/src/tenantq/search.py
index 9a51c6a..5923be5 100644
--- a/src/tenantq/search.py
+++ b/src/tenantq/search.py
@@ -44,6 +44,12 @@ def build_filter(
     created_before: Optional[int] = None,
 ) -> models.Filter:
     """Build a tenant-scoped filter, optionally narrowed by metadata."""
+    if tenant_id is None or not str(tenant_id).strip():
+        raise ValueError(
+            "tenant_id is required: request was not scoped to a tenant "
+            "(empty or whitespace-only tenant_id)"
+        )
+    tenant_id = str(tenant_id).strip()
     must: List[models.FieldCondition] = [
         models.FieldCondition(key=TENANT_FIELD, match=models.MatchValue(value=tenant_id))
     ]
diff --git a/tests/test_search.py b/tests/test_search.py
index 8a936f8..7e7ebc6 100644
--- a/tests/test_search.py
+++ b/tests/test_search.py
@@ -66,3 +66,16 @@ def test_search_accepts_query_with_internal_spaces(ingested, settings, embedder)
         limit=5,
     )
     assert len(hits) > 0
+
+
+@pytest.mark.parametrize("bad", ["", "   ", "\t"])
+def test_build_filter_rejects_empty_tenant_id(bad):
+    from tenantq.search import build_filter
+    with pytest.raises(ValueError, match="not scoped"):
+        build_filter(bad)
+
+
+def test_build_filter_accepts_tenant_with_internal_spaces():
+    from tenantq.search import build_filter
+    f = build_filter("acme corp")
+    assert f.must