diff --git a/ext/erb/erb.c b/ext/erb/erb.c
index 1c3371d24e..a103fb30e4 100644
--- a/ext/erb/erb.c
+++ b/ext/erb/erb.c
@@ -76,7 +76,9 @@ optimized_escape_html(VALUE str)
 static VALUE
 erb_escape_html(VALUE self, VALUE str)
 {
-    str = rb_convert_type(str, T_STRING, "String", "to_s");
+    if (!RB_TYPE_P(str, T_STRING)) {
+        str = rb_convert_type(str, T_STRING, "String", "to_s");
+    }
 
     if (rb_enc_str_asciicompat_p(str)) {
         return optimized_escape_html(str);