diff --git a/shared/lib/Json/JsonSerializableObject.php b/shared/lib/Json/JsonSerializableObject.php
index b198dd0..ca0c7d8 100644
--- a/shared/lib/Json/JsonSerializableObject.php
+++ b/shared/lib/Json/JsonSerializableObject.php
@@ -77,7 +77,14 @@ abstract class JsonSerializableObject implements JsonSerializable, JsonDeseriali
 				if (!empty($this->serializableProperties) && !in_array($key, $this->serializableProperties)) {
 					continue;
 				}
-				
+
+				// Null means absent: leave the property at its default rather
+				// than passing null into an object's jsonDeserialize/enum
+				// coercion or overwriting an initialized collection.
+				if ($value === null) {
+					continue;
+				}
+
 				$type = gettype($this->$key);
 				
 				// Handle JsonDeserializable objects
@@ -101,9 +108,10 @@ abstract class JsonSerializableObject implements JsonSerializable, JsonDeseriali
 					$enumClass = get_class($this->$key);
 					$this->$key = $enumClass::from($value);
 				}
-				// Handle regular values
+				// Handle regular values, coercing by declared type when the
+				// current value is null and carries no instance to key off
 				else {
-					$this->$key = $value;
+					$this->$key = $this->coerceValue($key, $value);
 				}
 			}
 		}
@@ -111,6 +119,48 @@ abstract class JsonSerializableObject implements JsonSerializable, JsonDeseriali
 		return $this;
 	}
 
+	/**
+	 * Coerces a decoded JSON value to the property's declared type. Needed for
+	 * properties whose current value is null: the instanceof-based branches
+	 * above cannot dispatch without an instance, and assigning the raw string
+	 * to a typed property (DateTime, enum, ...) would raise a TypeError.
+	 */
+	private function coerceValue(string $key, mixed $value): mixed {
+		$type = (new \ReflectionProperty($this, $key))->getType();
+		if ($type instanceof \ReflectionNamedType) {
+			$names = [$type->getName()];
+		} elseif ($type instanceof \ReflectionUnionType) {
+			$names = array_map(static fn($member) => $member->getName(), $type->getTypes());
+		} else {
+			return $value;
+		}
+
+		foreach ($names as $name) {
+			if (!class_exists($name) && !interface_exists($name) && !enum_exists($name)) {
+				continue;
+			}
+			if (is_string($value) && is_a($name, DateTimeInterface::class, true)) {
+				return $name === \DateTime::class ? new \DateTime($value) : new \DateTimeImmutable($value);
+			}
+			if (is_string($value) && is_a($name, DateTimeZone::class, true)) {
+				return new DateTimeZone($value);
+			}
+			if (is_string($value) && is_a($name, DateInterval::class, true)) {
+				return $this->toDateInterval($value);
+			}
+			if ((is_string($value) || is_int($value)) && is_subclass_of($name, \BackedEnum::class)) {
+				return $name::from($value);
+			}
+			if ((is_array($value) || is_string($value)) && is_subclass_of($name, JsonDeserializable::class)) {
+				/** @var JsonDeserializable $instance */
+				$instance = new $name();
+				return $instance->jsonDeserialize($value);
+			}
+		}
+
+		return $value;
+	}
+
 	protected function fromDateInterval(DateInterval $interval): string {
 		$spec = '';