diff --git a/.changeset/present-properties-match.md b/.changeset/present-properties-match.md
new file mode 100644
index 0000000..2e34024
--- /dev/null
+++ b/.changeset/present-properties-match.md
@@ -0,0 +1,5 @@
+---
+"posthog-php": patch
+---
+
+Return false for the `is_not_set` operator during local evaluation when the property key is present.
diff --git a/lib/FeatureFlag.php b/lib/FeatureFlag.php
index 55f6e4c..3bccd05 100644
--- a/lib/FeatureFlag.php
+++ b/lib/FeatureFlag.php
@@ -32,7 +32,7 @@ public static function matchProperty($property, $propertyValues)
         }
 
         if ($operator == "is_not_set") {
-            throw new InconclusiveMatchException("can't match properties with operator is_not_set");
+            return false;
         }
 
         $overrideValue = $propertyValues[$key];
diff --git a/test/FeatureFlagLocalEvaluationTest.php b/test/FeatureFlagLocalEvaluationTest.php
index ee7064b..7809088 100644
--- a/test/FeatureFlagLocalEvaluationTest.php
+++ b/test/FeatureFlagLocalEvaluationTest.php
@@ -248,34 +248,55 @@ public function testMatchPropertyNotIn(): void
         ]);
     }
 
-    public function testMatchPropertyIsSet(): void
+    /**
+     * @dataProvider presentPresenceOperatorValuesProvider
+     */
+    public function testMatchPropertyPresenceOperatorsTreatPresentValuesAsSet($value): void
     {
-        $prop = [
-            "key" => "key",
-            "value" => "is_set",
-            "operator" => "is_set"
-        ];
-
-        self::assertTrue(FeatureFlag::matchProperty($prop, [
-            "key" => "value",
-        ]));
+        foreach (["is_set" => true, "is_not_set" => false] as $operator => $expected) {
+            $prop = [
+                "key" => "key",
+                "value" => $operator,
+                "operator" => $operator,
+            ];
 
-        self::assertTrue(FeatureFlag::matchProperty($prop, [
-            "key" => "value2",
-        ]));
+            self::assertSame($expected, FeatureFlag::matchProperty($prop, [
+                "key" => $value,
+            ]));
+        }
+    }
 
-        self::assertTrue(FeatureFlag::matchProperty($prop, [
-            "key" => "",
-        ]));
+    public static function presentPresenceOperatorValuesProvider(): array
+    {
+        return [
+            'null' => [null],
+            'false' => [false],
+            'zero' => [0],
+            'empty string' => [''],
+            'empty array' => [[]],
+            // stdClass is PHP's JSON-object host equivalent and keeps it distinct from [] (an empty array).
+            'empty object' => [new \stdClass()],
+        ];
+    }
 
-        self::assertTrue(FeatureFlag::matchProperty($prop, [
-            "key" => null,
-        ]));
+    public function testMatchPropertyPresenceOperatorsAreInconclusiveForOmittedKey(): void
+    {
+        foreach (["is_set", "is_not_set"] as $operator) {
+            $prop = [
+                "key" => "key",
+                "value" => $operator,
+                "operator" => $operator,
+            ];
 
-        self::expectException(InconclusiveMatchException::class);
-        FeatureFlag::matchProperty($prop, [
-            "key2" => "value",
-        ]);
+            try {
+                FeatureFlag::matchProperty($prop, [
+                    "key2" => "value",
+                ]);
+                self::fail("Expected InconclusiveMatchException for operator {$operator}");
+            } catch (InconclusiveMatchException $exception) {
+                self::assertInstanceOf(InconclusiveMatchException::class, $exception);
+            }
+        }
     }
 
     public function testMatchPropertyContains(): void