1 files changed, 10 insertions, 1 deletions
diff --git a/string.c b/string.c
index 985c427786..d53476975d 100644
--- a/string.c
+++ b/string.c
@@ -5114,8 +5114,17 @@ rb_str_setter(VALUE val, ID id, VALUE *var)
 static VALUE
 rb_str_force_encoding(VALUE str, VALUE encname)
 {
+    const char *name;
+    int idx;
+
+    if (NIL_P(encname)) {
+	idx = 0;
+    }
+    else if ((idx = rb_enc_find_index(name = StringValueCStr(encname))) < 0) {
+	rb_raise(rb_eArgError, "invalid encoding name - %s", name);
+    }
     str_modifiable(str);
-    rb_enc_associate(str, rb_enc_find(StringValueCStr(encname)));
+    rb_enc_associate_index(str, idx);
     return str;
 }