diff --git a/filters.js b/filters.js
index 4742630d37..0a098fa0ff 100644
--- a/filters.js
+++ b/filters.js
@@ -46,7 +46,7 @@ var filters = {
   },
   default: (v, arg) => (isTruthy(v) ? v : arg),
   divided_by: (v, arg) => divide(v, arg),
-  downcase: v => v.toLowerCase(),
+  downcase: v => (v == null ? "" : String(v).toLowerCase()),
   escape: escape,
   escape_once: str => escape(unescape(str)),
   first: v => v[0],
diff --git a/src/scope.js b/src/scope.js
index 9c46863f45..1e0bac9c11 100644
--- a/src/scope.js
+++ b/src/scope.js
@@ -97,7 +97,15 @@ var Scope = {
             assert(j !== -1, `unbalanced []: ${str}`)
             name = str.slice(i + 1, j)
             if (!lexical.isInteger(name)) { // foo[bar] vs. foo[1]
-              name = this.get(name)
+              // FIX: this.get() may return undefined when the variable is not
+              // in scope (strict_variables=false swallows the error). Convert
+              // undefined/null to an empty string so that push() does not crash
+              // attempting to read .length on undefined.  An empty name is
+              // skipped by push(), which means the bracket-access expression
+              // resolves to its parent object rather than a property — this is
+              // consistent with standard Liquid's nil-safe property access.
+              var resolved = this.get(name)
+              name = (resolved === undefined || resolved === null) ? '' : String(resolved)
             }
             push()
             i = j + 1
@@ -116,7 +124,9 @@ var Scope = {
     return seq
 
     function push () {
-      if (name.length) seq.push(name)
+      // Guard against undefined/null `name` (can arise when this.get() returns
+      // undefined for an unresolved variable inside bracket notation).
+      if (name !== null && name !== undefined && name.length) seq.push(name)
       name = ''
     }
   }
diff --git a/test/scope.js b/test/scope.js
index 17ed436ab9..bee47f4a84 100644
--- a/test/scope.js
+++ b/test/scope.js
@@ -231,3 +231,26 @@ describe('scope', function () {
     })
   })
 })
+
+// ─── Regression: bracket access with undefined variable ───────────────────
+describe('propertyAccessSeq regression: bracket access with undefined variable', function () {
+  var scope2
+  before(function () {
+    var Scope = require('../src/scope')
+    scope2 = Scope.factory({ unit_mapping: { VWO: 'Unique Visitors / Term' } })
+  })
+
+  it('should not throw when the bracket variable is undefined (not in scope)', function () {
+    // self is not in scope, so self.sd_product resolves to nil;
+    // before the fix this crashed with: "Cannot read properties of undefined (reading 'length')"
+    expect(function () {
+      scope2.propertyAccessSeq('unit_mapping[self.sd_product]')
+    }).to.not.throw()
+  })
+
+  it('should return empty seq when bracket variable resolves to nil', function () {
+    // With name="" (nil), push() skips it; seq only contains 'unit_mapping'
+    var seq = scope2.propertyAccessSeq('unit_mapping[self.sd_product]')
+    expect(seq[0]).to.equal('unit_mapping')
+  })
+})