diff --git a/Puc/v5p3/Scheduler.php b/Puc/v5p3/Scheduler.php
index 05d743e..79f20f2 100644
--- a/Puc/v5p3/Scheduler.php
+++ b/Puc/v5p3/Scheduler.php
@@ -187,6 +187,21 @@ if ( !class_exists(Scheduler::class, false) ):
 			$state = $this->updateChecker->getUpdateState();
 			$shouldCheck = ($state->timeSinceLastCheck() >= $this->getEffectiveCheckPeriod());
 
+			if ( $shouldCheck ) {
+				//Sanity check: Do not proceed if one of the critical classes is missing.
+				//That can happen - theoretically and extremely rarely - if maybeCoreUpdate()
+				//is called before the old version of our plugin has been fully deleted, or
+				//called from an independent AJAX request during deletion.
+				if ( !(
+					class_exists(Utils::class)
+					&& class_exists(Metadata::class)
+					&& class_exists(Plugin\Update::class)
+					&& class_exists(Theme\Update::class)
+				) ) {
+					return;
+				}
+			}
+
 			//Let plugin authors substitute their own algorithm.
 			$shouldCheck = apply_filters(
 				$this->updateChecker->getUniqueName('check_now'),
diff --git a/Puc/v5p3/StateStore.php b/Puc/v5p3/StateStore.php
index f182787..1391111 100644
--- a/Puc/v5p3/StateStore.php
+++ b/Puc/v5p3/StateStore.php
@@ -163,7 +163,12 @@ if ( !class_exists(StateStore::class, false) ):
 
 			$state = get_site_option($this->optionName, null);
 
-			if ( !is_object($state) ) {
+			if (
+				!is_object($state)
+				//Sanity check: If the Utils class is missing, the plugin is probably in the process
+				//of being deleted (e.g. the old version gets deleted during an update).
+				|| !class_exists(Utils::class)
+			) {
 				$this->lastCheck = 0;
 				$this->checkedVersion = '';
 				$this->update = null;