diff --git a/ext/json/ext/fbuffer/fbuffer.h b/ext/json/ext/fbuffer/fbuffer.h
index 752d153b..8ce029a8 100644
--- a/ext/json/ext/fbuffer/fbuffer.h
+++ b/ext/json/ext/fbuffer/fbuffer.h
@@ -161,23 +161,25 @@ static inline void fbuffer_append_reserved_char(FBuffer *fb, char chr)
 
 static void fbuffer_append_str(FBuffer *fb, VALUE str)
 {
-    const char *newstr = StringValuePtr(str);
-    unsigned long len = RSTRING_LEN(str);
+    const char *ptr;
+    unsigned long len;
+    RSTRING_GETMEM(str, ptr, len);
 
-    fbuffer_append(fb, newstr, len);
+    fbuffer_append(fb, ptr, len);
 }
 
 static void fbuffer_append_str_repeat(FBuffer *fb, VALUE str, size_t repeat)
 {
-    const char *newstr = StringValuePtr(str);
-    unsigned long len = RSTRING_LEN(str);
+    const char *ptr;
+    unsigned long len;
+    RSTRING_GETMEM(str, ptr, len);
 
     fbuffer_inc_capa(fb, repeat * len);
     while (repeat) {
 #if JSON_DEBUG
         fb->requested = len;
 #endif
-        fbuffer_append_reserved(fb, newstr, len);
+        fbuffer_append_reserved(fb, ptr, len);
         repeat--;
     }
 }
diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index dbba99c4..4cbe43a3 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -1374,7 +1374,7 @@ static void generate_json_fixnum(FBuffer *buffer, struct generate_json_data *dat
 static void generate_json_bignum(FBuffer *buffer, struct generate_json_data *data, VALUE obj)
 {
     VALUE tmp = rb_funcall(obj, i_to_s, 0);
-    fbuffer_append_str(buffer, tmp);
+    fbuffer_append_str(buffer, StringValue(tmp));
 }
 
 #ifdef RUBY_INTEGER_UNIFICATION