diff --git a/isthmus/src/main/java/io/substrait/isthmus/OuterReferenceResolver.java b/isthmus/src/main/java/io/substrait/isthmus/OuterReferenceResolver.java
index 8071468c0..76b20c8e1 100644
--- a/isthmus/src/main/java/io/substrait/isthmus/OuterReferenceResolver.java
+++ b/isthmus/src/main/java/io/substrait/isthmus/OuterReferenceResolver.java
@@ -66,6 +66,27 @@ public RelNode visit(Filter filter) throws RuntimeException {
     for (CorrelationId id : filter.getVariablesSet()) {
       nestedDepth.putIfAbsent(id, 0);
     }
+    // Also register any CorrelationIds referenced directly in the condition expression.
+    // This covers partially-decorrelated plans where a Calcite optimizer (e.g. HepPlanner)
+    // rewrites correlated IN-subqueries into joins but leaves scalar-aggregate subquery
+    // correlations as RexFieldAccess($corN.col) inside a Filter condition without a
+    // surrounding Correlate node. In that case getVariablesSet() is empty but the condition
+    // still contains RexCorrelVariable references whose IDs must be in nestedDepth before
+    // rexVisitor.visitFieldAccess() is called.
+    filter
+        .getCondition()
+        .accept(
+            new RexShuttle() {
+              @Override
+              public RexNode visitFieldAccess(RexFieldAccess fieldAccess) {
+                if (fieldAccess.getReferenceExpr() instanceof RexCorrelVariable) {
+                  CorrelationId id =
+                      ((RexCorrelVariable) fieldAccess.getReferenceExpr()).id;
+                  nestedDepth.putIfAbsent(id, 0);
+                }
+                return fieldAccess;
+              }
+            });
     filter.getCondition().accept(rexVisitor);
     return super.visit(filter);
   }
diff --git a/isthmus/src/test/java/io/substrait/isthmus/OuterReferenceResolverTest.java b/isthmus/src/test/java/io/substrait/isthmus/OuterReferenceResolverTest.java
index a804aafb1..aa151b007 100644
--- a/isthmus/src/test/java/io/substrait/isthmus/OuterReferenceResolverTest.java
+++ b/isthmus/src/test/java/io/substrait/isthmus/OuterReferenceResolverTest.java
@@ -7,6 +7,7 @@
 import org.apache.calcite.rel.core.JoinRelType;
 import org.apache.calcite.rex.RexCorrelVariable;
 import org.apache.calcite.rex.RexFieldAccess;
+import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.sql.parser.SqlParseException;
 import org.apache.calcite.tools.RelBuilder;
 import org.apache.calcite.util.Holder;
@@ -142,4 +143,63 @@ void nestedApplyJoinQuery() throws SqlParseException {
     validateOuterRef(fieldAccessDepthMap, "$cor0", "SS_ITEM_SK", 2);
     validateOuterRef(fieldAccessDepthMap, "$cor1", "I_ITEM_SK", 1);
   }
+
+  /**
+   * Regression test for the partially-decorrelated Filter case.
+   *
+   * <p>When a Calcite optimizer (e.g. HepPlanner) rewrites a correlated IN-subquery into a join, it
+   * can produce a {@link org.apache.calcite.rel.core.Filter} whose condition contains a {@link
+   * RexFieldAccess} referencing a {@link RexCorrelVariable} ({@code $cor0.field}), but whose {@link
+   * RelNode#getVariablesSet()} is empty — because the enclosing {@link
+   * org.apache.calcite.rel.core.Correlate} node has already been replaced by a regular join.
+   *
+   * <p>Before the fix, {@code OuterReferenceResolver.visit(Filter)} only iterated {@code
+   * filter.getVariablesSet()}, so the {@link org.apache.calcite.rel.core.CorrelationId} was never
+   * registered in {@code nestedDepth}. The subsequent {@code rexVisitor.visitFieldAccess()} call
+   * found {@code !nestedDepth.containsKey(id)} and silently skipped the access, producing an empty
+   * {@code fieldAccessDepthMap} instead of the expected entry.
+   *
+   * <p>The fix pre-scans the Filter condition for {@link RexCorrelVariable} references and registers
+   * their IDs before delegating to the rex visitor.
+   */
+  @Test
+  void filterWithCorrelVariableButEmptyVariablesSet() throws SqlParseException {
+    // Build the partially-decorrelated pattern:
+    //   JOIN (inner side has a Filter whose condition references $cor0 but variablesSet is empty)
+    //
+    // SQL equivalent of what a post-optimisation plan looks like:
+    //   SELECT ss_sold_date_sk FROM store_sales
+    //   JOIN item ON item.i_item_sk = store_sales.ss_item_sk   ← decorrelated join
+    //   WHERE item.i_item_sk = $cor0.SS_ITEM_SK               ← Filter still has the correl ref
+    //
+    // We deliberately call .filter(condition) without passing the CorrelationId so that
+    // getVariablesSet() returns an empty set, reproducing the post-decorrelation shape.
+
+    final Holder<RexCorrelVariable> cor0 = Holder.empty();
+
+    // Push STORE_SALES and capture the correlation variable for it.
+    tpcDsRelBuilder.scan("tpcds", "STORE_SALES").variable(cor0::set);
+
+    // Push ITEM on top, then build the filter condition while ITEM is the top-of-stack.
+    // The condition equates item.I_ITEM_SK to the correlated $cor0.SS_ITEM_SK.
+    tpcDsRelBuilder.scan("tpcds", "ITEM");
+    RexNode correlCondition =
+        tpcDsRelBuilder.equals(
+            tpcDsRelBuilder.field("I_ITEM_SK"),
+            tpcDsRelBuilder.field(cor0.get(), "SS_ITEM_SK"));
+
+    final RelNode calciteRel =
+        tpcDsRelBuilder
+            // Intentionally NOT passing cor0.get().id to filter() → getVariablesSet() is empty.
+            .filter(correlCondition)
+            .join(JoinRelType.INNER, tpcDsRelBuilder.literal(true))
+            .project(tpcDsRelBuilder.field("SS_SOLD_DATE_SK"))
+            .build();
+
+    // Pre-fix: fieldAccessDepthMap would be empty because the CorrelationId was never registered.
+    // Post-fix: the Filter condition pre-scan registers $cor0, so the field access is recorded.
+    final Map<RexFieldAccess, Integer> fieldAccessDepthMap = buildOuterFieldRefMap(calciteRel);
+    Assertions.assertEquals(1, fieldAccessDepthMap.size());
+    validateOuterRef(fieldAccessDepthMap, "$cor0", "SS_ITEM_SK", 0);
+  }
 }