diff --git a/Classes/SBJsonStreamParserState.m b/Classes/SBJsonStreamParserState.m
index f0cd74a6..42981ad9 100644
--- a/Classes/SBJsonStreamParserState.m
+++ b/Classes/SBJsonStreamParserState.m
@@ -36,7 +36,11 @@
 #define SINGLETON \
 + (id)sharedInstance { \
     static id state = nil; \
-    if (!state) state = [[self alloc] init]; \
+    if (!state) { \
+        @synchronized(self) { \
+            if (!state) state = [[self alloc] init]; \
+        } \
+    } \
     return state; \
 }
 
diff --git a/Classes/SBJsonStreamWriterState.m b/Classes/SBJsonStreamWriterState.m
index 9f04cac4..8eb47f36 100644
--- a/Classes/SBJsonStreamWriterState.m
+++ b/Classes/SBJsonStreamWriterState.m
@@ -35,8 +35,12 @@
 
 #define SINGLETON \
 + (id)sharedInstance { \
-    static id state; \
-    if (!state) state = [[self alloc] init]; \
+    static id state = nil; \
+    if (!state) { \
+        @synchronized(self) { \
+            if (!state) state = [[self alloc] init]; \
+        } \
+    } \
     return state; \
 }