diff --git a/src/gateway/server.py b/src/gateway/server.py
index 4c656ee..8e2135d 100644
--- a/src/gateway/server.py
+++ b/src/gateway/server.py
@@ -345,6 +345,8 @@ def AnalyzePlan(self, request, context):
         if request.schema:
             substrait = self._converter.convert_plan(request.schema.plan)
             self._statistics.add_plan(substrait)
+            if len(substrait.relations) != 1:
+                raise ValueError(f"Expected exactly _ONE_ relation in the plan: {request}")
             try:
                 results = self._backend.execute(substrait)
             except Exception as err: