diff --git a/src/Database/Validator/Query/Filter.php b/src/Database/Validator/Query/Filter.php
index ca124ec28..50427c015 100644
--- a/src/Database/Validator/Query/Filter.php
+++ b/src/Database/Validator/Query/Filter.php
@@ -316,7 +316,8 @@ protected function isEmpty(array $values): bool
             return true;
         }
 
-        if (is_array($values[0]) && count($values[0]) === 0) {
+        $firstKey = \array_key_first($values);
+        if (\is_array($values[$firstKey]) && \count($values[$firstKey]) === 0) {
             return true;
         }
 
diff --git a/tests/unit/Validator/Query/FilterTest.php b/tests/unit/Validator/Query/FilterTest.php
index 21ba8f404..4bdff1270 100644
--- a/tests/unit/Validator/Query/FilterTest.php
+++ b/tests/unit/Validator/Query/FilterTest.php
@@ -122,6 +122,16 @@ public function testEmptyValues(): void
         $this->assertEquals('Equal queries require at least one value.', $this->validator->getDescription());
     }
 
+    public function testIsEmptyHandlesNonZeroIndexedValues(): void
+    {
+        // Sparse / associative arrays must not trigger "Undefined array key 0".
+        $sparse = new Query(Query::TYPE_EQUAL, 'string', [1 => 'super']);
+        $this->assertTrue($this->validator->isValid($sparse));
+
+        $assoc = new Query(Query::TYPE_CONTAINS, 'string_array', ['foo' => 'super']);
+        $this->assertTrue($this->validator->isValid($assoc));
+    }
+
     public function testMaxValuesCount(): void
     {
         $max = $this->validator->getMaxValuesCount();