diff --git a/rhino/src/main/java/org/mozilla/javascript/ScriptRuntime.java b/rhino/src/main/java/org/mozilla/javascript/ScriptRuntime.java
index ea7be1f238..1c3ac25ab7 100644
--- a/rhino/src/main/java/org/mozilla/javascript/ScriptRuntime.java
+++ b/rhino/src/main/java/org/mozilla/javascript/ScriptRuntime.java
@@ -3737,7 +3737,15 @@ public static boolean eq(Object x, Object y) {
             if (isSymbol(y) && isObject(x)) {
                 return eq(toPrimitive(x), y);
             }
-            if (y instanceof Scriptable) {
+            if (y == null || Undefined.isUndefined(y)) {
+                if (x instanceof ScriptableObject) {
+                    Object test = ((ScriptableObject) x).equivalentValues(y);
+                    if (test != Scriptable.NOT_FOUND) {
+                        return ((Boolean) test).booleanValue();
+                    }
+                }
+                return false;
+            } else if (y instanceof Scriptable) {
                 if (x instanceof ScriptableObject) {
                     Object test = ((ScriptableObject) x).equivalentValues(y);
                     if (test != Scriptable.NOT_FOUND) {
diff --git a/rhino/src/main/java/org/mozilla/javascript/optimizer/BodyCodegen.java b/rhino/src/main/java/org/mozilla/javascript/optimizer/BodyCodegen.java
index f4432f9c97..6bb3a000de 100644
--- a/rhino/src/main/java/org/mozilla/javascript/optimizer/BodyCodegen.java
+++ b/rhino/src/main/java/org/mozilla/javascript/optimizer/BodyCodegen.java
@@ -3934,34 +3934,18 @@ private void visitIfJumpEqOp(Node node, Node child, int trueGOTO, int falseGOTO)
         int type = node.getType();
         Node rChild = child.getNext();
 
-        // Optimize if one of operands is null
-        if (child.getType() == Token.NULL || rChild.getType() == Token.NULL) {
+        // Optimize if one of operands is null; but we can't do this
+        // for EQ/NEQ because a ScripableObject might overwrite equivalentValues()
+        if (type != Token.EQ
+                && type != Token.NE
+                && (child.getType() == Token.NULL || rChild.getType() == Token.NULL)) {
             // eq is symmetric in this case
             if (child.getType() == Token.NULL) {
                 child = rChild;
             }
             generateExpression(child, node);
-            if (type == Token.SHEQ || type == Token.SHNE) {
-                int testCode = (type == Token.SHEQ) ? ByteCode.IFNULL : ByteCode.IFNONNULL;
-                cfw.add(testCode, trueGOTO);
-            } else {
-                if (type != Token.EQ) {
-                    // swap false/true targets for !=
-                    if (type != Token.NE) throw Codegen.badTree();
-                    int tmp = trueGOTO;
-                    trueGOTO = falseGOTO;
-                    falseGOTO = tmp;
-                }
-                cfw.add(ByteCode.DUP);
-                int undefCheckLabel = cfw.acquireLabel();
-                cfw.add(ByteCode.IFNONNULL, undefCheckLabel);
-                int stack = cfw.getStackTop();
-                cfw.add(ByteCode.POP);
-                cfw.add(ByteCode.GOTO, trueGOTO);
-                cfw.markLabel(undefCheckLabel, stack);
-                Codegen.pushUndefined(cfw);
-                cfw.add(ByteCode.IF_ACMPEQ, trueGOTO);
-            }
+            int testCode = (type == Token.SHEQ) ? ByteCode.IFNULL : ByteCode.IFNONNULL;
+            cfw.add(testCode, trueGOTO);
             cfw.add(ByteCode.GOTO, falseGOTO);
         } else {
             int child_dcp_register = nodeIsDirectCallParameter(child);
diff --git a/rhino/src/test/java/org/mozilla/javascript/tests/ScriptRuntimeEquivalentValuesTest.java b/rhino/src/test/java/org/mozilla/javascript/tests/ScriptRuntimeEquivalentValuesTest.java
new file mode 100644
index 0000000000..7dedbf2dad
--- /dev/null
+++ b/rhino/src/test/java/org/mozilla/javascript/tests/ScriptRuntimeEquivalentValuesTest.java
@@ -0,0 +1,86 @@
+package org.mozilla.javascript.tests;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+import org.mozilla.javascript.*;
+import org.mozilla.javascript.annotations.JSConstructor;
+
+/**
+ * Test cases for the {@link ScriptRuntime} support for ScriptableObject#equivalentValues(Object)
+ * method.
+ *
+ * @author Ronald Brill
+ */
+public class ScriptRuntimeEquivalentValuesTest {
+
+    @Test
+    public void equivalentValuesUndefined() throws Exception {
+        Utils.runWithAllOptimizationLevels(
+                cx -> {
+                    final Scriptable scope = cx.initStandardObjects();
+                    try {
+                        ScriptableObject.defineClass(scope, EquivalentTesterObject.class);
+                    } catch (Exception e) {
+                    }
+
+                    Object result =
+                            cx.evaluateString(
+                                    scope,
+                                    "var o = new EquivalentTesterObject();"
+                                            + "'' + (o == undefined) + ' ' + (undefined == o)",
+                                    "test",
+                                    1,
+                                    null);
+                    assertEquals("" + cx.getOptimizationLevel(), "true true", result);
+
+                    return null;
+                });
+    }
+
+    @Test
+    public void equivalentValuesNull() throws Exception {
+        Utils.runWithAllOptimizationLevels(
+                cx -> {
+                    final Scriptable scope = cx.initStandardObjects();
+                    try {
+                        ScriptableObject.defineClass(scope, EquivalentTesterObject.class);
+                    } catch (Exception e) {
+                    }
+
+                    Object result =
+                            cx.evaluateString(
+                                    scope,
+                                    "var o = new EquivalentTesterObject();"
+                                            + "'' + (o == null) + ' ' + (null == o)",
+                                    "test",
+                                    1,
+                                    null);
+                    assertEquals("" + cx.getOptimizationLevel(), "true true", result);
+
+                    return null;
+                });
+    }
+
+    public static class EquivalentTesterObject extends ScriptableObject {
+
+        public EquivalentTesterObject() {}
+
+        @Override
+        public String getClassName() {
+            return "EquivalentTesterObject";
+        }
+
+        @JSConstructor
+        public void jsConstructorMethod() {}
+
+        @Override
+        protected Object equivalentValues(final Object value) {
+            if (value == null || Undefined.isUndefined(value)) {
+                return Boolean.TRUE;
+            }
+
+            return super.equivalentValues(value);
+        }
+    }
+}