diff --git a/ext/standard/array.c b/ext/standard/array.c
index f8dd7d891dd3..570e9e30e0da 100644
--- a/ext/standard/array.c
+++ b/ext/standard/array.c
@@ -4863,6 +4863,18 @@ PHP_FUNCTION(array_unique)
 		return;
 	}
 
+	zend_long base_sort = sort_type & ~PHP_SORT_FLAG_CASE;
+
+	if (base_sort != PHP_SORT_REGULAR
+	 && base_sort != PHP_SORT_NUMERIC
+	 && base_sort != PHP_SORT_STRING
+	 && base_sort != PHP_SORT_LOCALE_STRING) {
+
+		zend_argument_value_error(
+			2, "must be one of SORT_REGULAR, SORT_NUMERIC, SORT_STRING, or SORT_LOCALE_STRING");
+		RETURN_THROWS();
+	}
+
 	if (sort_type == PHP_SORT_STRING) {
 		HashTable seen;
 		zend_long num_key;
diff --git a/ext/standard/tests/array/array_unique_invalid_sort_type.phpt b/ext/standard/tests/array/array_unique_invalid_sort_type.phpt
new file mode 100644
index 000000000000..e431b66700ef
--- /dev/null
+++ b/ext/standard/tests/array/array_unique_invalid_sort_type.phpt
@@ -0,0 +1,20 @@
+--TEST--
+array_unique() throws ValueError on invalid sort_type
+--FILE--
+<?php
+try {
+    array_unique([1, 2, 3], 999);
+} catch (ValueError $e) {
+    echo $e->getMessage(), PHP_EOL;
+}
+
+try {
+    array_unique([1, 2, 3], -1);
+} catch (ValueError $e) {
+    echo $e->getMessage(), PHP_EOL;
+}
+?>
+--EXPECT--
+array_unique(): Argument #2 ($flags) must be one of SORT_REGULAR, SORT_NUMERIC, SORT_STRING, or SORT_LOCALE_STRING
+array_unique(): Argument #2 ($flags) must be one of SORT_REGULAR, SORT_NUMERIC, SORT_STRING, or SORT_LOCALE_STRING
+