diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 47c2c50b95..6ae9e805b5 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -810,11 +810,11 @@ strio_ungetbyte(VALUE self, VALUE c)
         return Qnil;
       case T_FIXNUM:
       case T_BIGNUM: ;
-        /* rb_int_modulo() not visible from exts */
-        VALUE v = rb_funcall(c, rb_intern("modulo"), 1, INT2FIX(256));
-        unsigned char cc = NUM2INT(v) & 0xFF;
-        c = rb_str_new((const char *)&cc, 1);
-        break;
+        /* rb_int_and() not visible from exts */
+        VALUE v = rb_funcall(c, '&', 1, INT2FIX(0xff));
+        const char cc = NUM2INT(v) & 0xFF;
+        strio_unget_bytes(ptr, &cc, 1);
+        return Qnil;
       default:
         SafeStringValue(c);
     }
diff --git a/string.c b/string.c
index 2fbe36b1b8..ccf4796d3b 100644
--- a/string.c
+++ b/string.c
@@ -5421,7 +5421,7 @@ rb_str_setbyte(VALUE str, VALUE index, VALUE value)
         pos += len;
 
     VALUE v = rb_to_int(value);
-    VALUE w = rb_int_modulo(v, INT2FIX(256));
+    VALUE w = rb_int_and(v, INT2FIX(0xff));
     unsigned char byte = NUM2INT(w) & 0xFF;
 
     if (!str_independent(str))