diff --git a/lib/rb/ext/binary_protocol_accelerated.c b/lib/rb/ext/binary_protocol_accelerated.c
index 728a057..75aa4c6 100644
--- a/lib/rb/ext/binary_protocol_accelerated.c
+++ b/lib/rb/ext/binary_protocol_accelerated.c
@@ -76,6 +76,9 @@
 }
 
 static void write_string_direct(VALUE trans, VALUE str) {
+  if (TYPE(str) != T_STRING) {
+    rb_raise(rb_eStandardError, "Value should be a string");    
+  }
   write_i32_direct(trans, RSTRING_LEN(str));
   rb_funcall(trans, write_method_id, 1, str);
 }