diff --git a/NSString+Score.m b/NSString+Score.m
index 6f6cdb8..41ff5f6 100644
--- a/NSString+Score.m
+++ b/NSString+Score.m
@@ -20,7 +20,7 @@ - (CGFloat) scoreAgainst:(NSString *)otherString fuzziness:(NSNumber *)fuzziness
 }
 
 - (CGFloat) scoreAgainst:(NSString *)anotherString fuzziness:(NSNumber *)fuzziness options:(NSStringScoreOption)options{
-    NSMutableCharacterSet *workingInvalidCharacterSet = [NSCharacterSet lowercaseLetterCharacterSet];
+    NSMutableCharacterSet *workingInvalidCharacterSet = [[NSCharacterSet lowercaseLetterCharacterSet] mutableCopy];
     [workingInvalidCharacterSet formUnionWithCharacterSet:[NSCharacterSet uppercaseLetterCharacterSet]];
     [workingInvalidCharacterSet addCharactersInString:@" "];
     NSCharacterSet *invalidCharacterSet = [workingInvalidCharacterSet invertedSet];