diff --git a/ChangeLog b/ChangeLog
index bdcdbd25c1..bc95812ae8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed May  8 13:49:38 2013  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* ext/json/generator/generator.c (isArrayOrObject): cast char to
+	  unsigned char. [Bug #8378]
+
 Wed May  8 13:46:10 2013  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* ext/json/generator/depend: fix dependencies [Bug #8379]
diff --git a/ext/json/generator/generator.c b/ext/json/generator/generator.c
index 42a1658811..7bc193455d 100644
--- a/ext/json/generator/generator.c
+++ b/ext/json/generator/generator.c
@@ -894,8 +894,8 @@ static int isArrayOrObject(VALUE string)
     long string_len = RSTRING_LEN(string);
     char *p = RSTRING_PTR(string), *q = p + string_len - 1;
     if (string_len < 2) return 0;
-    for (; p < q && isspace(*p); p++);
-    for (; q > p && isspace(*q); q--);
+    for (; p < q && isspace((unsigned char)*p); p++);
+    for (; q > p && isspace((unsigned char)*q); q--);
     return (*p == '[' && *q == ']') || (*p == '{' && *q == '}');
 }