diff --git a/ChangeLog b/ChangeLog
index 28a684539a..471bf84e16 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Jun 21 16:38:14 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_casemap): do not put code with side effects
+	  inside RSTRING_PTR() macro which evaluates the argument multiple
+	  times.
+
 Tue Jun 21 16:13:45 2016  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* string.c (rb_str_casemap): fix memory leak.
diff --git a/string.c b/string.c
index a62ae2dd8d..e4f0293bb8 100644
--- a/string.c
+++ b/string.c
@@ -5811,12 +5811,14 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
 
     if (buffer_count==1) {
 	target = rb_str_new_with_class(source, (const char*)current_buffer->space, target_length);
-	    xfree(current_buffer);
+	xfree(current_buffer);
     }
     else {
-	char *target_current = RSTRING_PTR(target = rb_str_new_with_class(source, 0, target_length));
+	char *target_current;
 	mapping_buffer *previous_buffer;
 
+	target = rb_str_new_with_class(source, 0, target_length);
+	target_current = RSTRING_PTR(target);
 	current_buffer=pre_buffer.next;
 	while (current_buffer) {
 	    memcpy(target_current, current_buffer->space, current_buffer->used);