diff --git a/lib/Property/ICalendar/Recur.php b/lib/Property/ICalendar/Recur.php
index 9a1e9e643..b229e87ab 100644
--- a/lib/Property/ICalendar/Recur.php
+++ b/lib/Property/ICalendar/Recur.php
@@ -189,7 +189,14 @@ public static function stringToArray(string $value): array
             if (empty($part)) {
                 continue;
             }
-            list($partName, $partValue) = explode('=', $part);
+
+            $parts = explode('=', $part);
+
+            if (count($parts) !== 2) {
+                throw new InvalidDataException('The supplied iCalendar RRULE part is incorrect: '.$part);
+            }
+
+            list($partName, $partValue) = $parts;
 
             // The value itself had multiple values..
             if (false !== strpos($partValue, ',')) {