diff --git a/packages/imhotep-solver/src/topology.ts b/packages/imhotep-solver/src/topology.ts
index 507f062..fd58627 100644
--- a/packages/imhotep-solver/src/topology.ts
+++ b/packages/imhotep-solver/src/topology.ts
@@ -33,11 +33,16 @@ function result(
 }
 
 /**
- * Safely read a topology value indexed by subject id.
- * Returns 0 when the subject is out of bounds (treated as "no relation").
+ * Safely read a topology value for a given solver subject ID.
+ *
+ * Solver subject IDs are 1-indexed; topology arrays are 0-indexed parallel
+ * to world.subjects.ids.  Returns 0 when the subject is not found (treated as
+ * "no relation").
  */
-function getTopologyValue(arr: number[], subjectId: number): number {
-  return arr[subjectId] ?? 0;
+function getTopologyValue(world: GeometryWorld, arr: number[], subjectId: number): number {
+  const idx = world.subjects.ids.indexOf(subjectId);
+  if (idx < 0 || idx >= arr.length) return 0;
+  return arr[idx];
 }
 
 /**
@@ -109,6 +114,7 @@ export function evaluateAttachedToScrollContainer(
   }
 
   const scrollContainerId = getTopologyValue(
+    world,
     world.topology.scrollContainerOf,
     subjectRef,
   );
@@ -139,7 +145,7 @@ export function evaluateInStackingContext(
     });
   }
 
-  const sCtx = getTopologyValue(world.topology.stackingContextOf, subjectRef);
+  const sCtx = getTopologyValue(world, world.topology.stackingContextOf, subjectRef);
 
   if (referenceRef === undefined) {
     const pass = sCtx !== 0;
@@ -152,7 +158,7 @@ export function evaluateInStackingContext(
     );
   }
 
-  const rCtx = getTopologyValue(world.topology.stackingContextOf, referenceRef);
+  const rCtx = getTopologyValue(world, world.topology.stackingContextOf, referenceRef);
   const pass = sCtx !== 0 && sCtx === rCtx;
 
   return result(