diff --git a/NEWS b/NEWS
index 6416ddedb98..81d0a689972 100644
--- a/NEWS
+++ b/NEWS
@@ -13,6 +13,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-21023 (CURLOPT_XFERINFOFUNCTION crash with a null callback).
     (David Carlier)
 
+- Date:
+  . Fixed bug GH-20936 (DatePeriod::__set_state() cannot handle null start).
+    (ndossche)
+
 - DOM:
   . Fixed bug GH-21077 (Accessing Dom\Node::baseURI can throw TypeError).
     (ndossche)
diff --git a/ext/date/php_date.c b/ext/date/php_date.c
index 6d0f0428f08..a67a58bcf92 100644
--- a/ext/date/php_date.c
+++ b/ext/date/php_date.c
@@ -5811,7 +5811,7 @@ static bool php_date_period_initialize_from_hash(php_period_obj *period_obj, Has
 			php_date_obj *date_obj;
 			date_obj = Z_PHPDATE_P(ht_entry);
 
-			if (!date_obj->time) {
+			if (!date_obj->time || !period_obj->start_ce) {
 				return 0;
 			}
 
@@ -5832,7 +5832,7 @@ static bool php_date_period_initialize_from_hash(php_period_obj *period_obj, Has
 			php_date_obj *date_obj;
 			date_obj = Z_PHPDATE_P(ht_entry);
 
-			if (!date_obj->time) {
+			if (!date_obj->time || !period_obj->start_ce) {
 				return 0;
 			}
 
diff --git a/ext/date/tests/gh20936.phpt b/ext/date/tests/gh20936.phpt
new file mode 100644
index 00000000000..e6a525dd458
--- /dev/null
+++ b/ext/date/tests/gh20936.phpt
@@ -0,0 +1,14 @@
+--TEST--
+GH-20936 (DatePeriod::__set_state() cannot handle null start)
+--FILE--
+<?php
+$end = new DateTime('2022-07-16', new DateTimeZone('UTC'));
+$interval = new DateInterval('P2D');
+try {
+    DatePeriod::__set_state(['start' => null, 'end' => $end, 'current' => null, 'interval' => $interval, 'recurrences' => 2, 'include_start_date' => false, 'include_end_date' => true]);
+} catch (Throwable $e) {
+    echo $e::class, ": ", $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+Error: Invalid serialization data for DatePeriod object