diff --git a/package.json b/package.json
index 2cffc26250..47916c3171 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "liquidjs",
-  "version": "3.1.0",
+  "version": "3.1.1",
   "description": "Liquid template engine by pure JavaScript: compatible to shopify, easy to extend.",
   "main": "index.js",
   "scripts": {
diff --git a/sd-custom/custom-operator/equals.js b/sd-custom/custom-operator/equals.js
index d9672bf3b9..1e509fb04f 100644
--- a/sd-custom/custom-operator/equals.js
+++ b/sd-custom/custom-operator/equals.js
@@ -95,7 +95,32 @@ function arrayEquals(l, r) {
   });
 }
 
+/**
+ * Default equality handler for primitive values.
+ *
+ * Treats `null` and `undefined` as equivalent — both represent an absent/nil value.
+ * This aligns with JavaScript's loose-equality semantics for nil (`null == undefined`)
+ * and fixes the IS_PRESENT computation bug where a questionnaire field initialised as
+ * `null` (never filled) was being treated as "present" because:
+ *   `NIL` (undefined variable) → `undefined`
+ *   `null !== undefined`        → `true`  ← incorrect: null should equal NIL
+ *
+ * Examples:
+ *   defaultHandler(null, undefined)  → true  (both are nil)
+ *   defaultHandler(null, null)       → true
+ *   defaultHandler(undefined, undefined) → true
+ *   defaultHandler(0, null)          → false  (0 is a real value, not nil)
+ *   defaultHandler(0, false)         → false  (unchanged strict equality)
+ */
 function defaultHandler(l, r) {
+  const lIsNil = l === null || l === undefined;
+  const rIsNil = r === null || r === undefined;
+
+  // Both nil → equal
+  if (lIsNil && rIsNil) return true;
+  // Only one nil → not equal
+  if (lIsNil !== rIsNil) return false;
+  // Neither nil → strict equality
   return l === r;
 }
 
diff --git a/test/sd-custom/custom-operator/equals.js b/test/sd-custom/custom-operator/equals.js
index 2493ba70c3..2dcbad271e 100644
--- a/test/sd-custom/custom-operator/equals.js
+++ b/test/sd-custom/custom-operator/equals.js
@@ -159,3 +159,32 @@ describe("Custom equals operator", () => {
     });
   });
 });
+
+  describe("nil equality — null and undefined should be treated as equal", () => {
+    it("should return true for null == null", () => {
+      expect(equals(null, null)).to.equal(true);
+    });
+    it("should return true for undefined == undefined", () => {
+      expect(equals(undefined, undefined)).to.equal(true);
+    });
+    it("should return true for null == undefined (NIL comparison fix)", () => {
+      // NIL in LiquidJS resolves to `undefined` (undefined variable).
+      // A questionnaire field that was never filled is sent as `null` by Django.
+      // Without this fix, `null != NIL` evaluated to `true` causing IS_PRESENT
+      // to incorrectly return true for blank fields.
+      expect(equals(null, undefined)).to.equal(true);
+      expect(equals(undefined, null)).to.equal(true);
+    });
+    it("should return false for null == 0 (0 is a real number value, not nil)", () => {
+      expect(equals(null, 0)).to.equal(false);
+      expect(equals(0, null)).to.equal(false);
+    });
+    it("should return false for null == false (false is a real boolean value, not nil)", () => {
+      expect(equals(null, false)).to.equal(false);
+      expect(equals(false, null)).to.equal(false);
+    });
+    it("should return false for null == empty string (empty string is not nil)", () => {
+      expect(equals(null, "")).to.equal(false);
+      expect(equals("", null)).to.equal(false);
+    });
+  });