diff --git a/src/PKTokenizer.m b/src/PKTokenizer.m
index 1d1cafd..63c1eb2 100644
--- a/src/PKTokenizer.m
+++ b/src/PKTokenizer.m
@@ -145,8 +145,12 @@ - (instancetype)initWithString:(NSString *)str stream:(NSInputStream *)stm {
 
 
 - (void)dealloc {
-    self.string = nil;
-    self.stream = nil;
+    if (self.string != nil) {
+        self.string = nil;
+    }
+    if (self.stream != nil) {
+        self.stream = nil;
+    }
     self.reader = nil;
     self.tokenizerStates = nil;
     self.numberState = nil;