diff --git a/Sources/HTMLCharacterData.m b/Sources/HTMLCharacterData.m
index 86e4f90..0891f64 100644
--- a/Sources/HTMLCharacterData.m
+++ b/Sources/HTMLCharacterData.m
@@ -91,7 +91,7 @@ NS_INLINE void CheckValidOffset(HTMLCharacterData *node, NSUInteger offset, NSSt
 - (id)copyWithZone:(NSZone *)zone
 {
 	HTMLCharacterData *copy = [super copyWithZone:zone];
-	copy->_data = _data;
+	copy->_data = [_data mutableCopy];
 	return copy;
 }
 
diff --git a/Sources/HTMLDocumentType.m b/Sources/HTMLDocumentType.m
index c22be69..0bff20c 100644
--- a/Sources/HTMLDocumentType.m
+++ b/Sources/HTMLDocumentType.m
@@ -139,8 +139,8 @@ NS_INLINE BOOL nilOrEqual(id first, id second) {
 - (id)copyWithZone:(NSZone *)zone
 {
 	HTMLDocumentType *copy = [super copyWithZone:zone];
-	copy->_publicIdentifier = self.publicIdentifier;
-	copy->_systemIdentifier = self.systemIdentifier;
+	copy->_publicIdentifier = [_publicIdentifier copy];
+	copy->_systemIdentifier = [_systemIdentifier copy];
 	return copy;
 }