diff --git a/time.c b/time.c
index da8feb7bf3..ddd011ed79 100644
--- a/time.c
+++ b/time.c
@@ -4145,7 +4145,7 @@ time_inspect(VALUE time)
     }
     else if (FIXNUM_P(subsec) && FIX2LONG(subsec) < TIME_SCALE) {
         long len;
-        str = rb_enc_sprintf(rb_usascii_encoding(), "%"PRIsVALUE".%09ld", str, FIX2LONG(subsec));
+        rb_str_catf(str, ".%09ld", FIX2LONG(subsec));
         for (len=RSTRING_LEN(str); RSTRING_PTR(str)[len-1] == '0' && len > 0; len--)
             ;
         rb_str_resize(str, len);