2 files changed, 9 insertions, 1 deletions
diff --git a/spec/ruby/optional/capi/ext/string_spec.c b/spec/ruby/optional/capi/ext/string_spec.c
index 070a88759b..cec3f65f45 100644
--- a/spec/ruby/optional/capi/ext/string_spec.c
+++ b/spec/ruby/optional/capi/ext/string_spec.c
@@ -573,7 +573,7 @@ static VALUE string_spec_rb_str_unlocktmp(VALUE self, VALUE str) {
 }
 
 static VALUE string_spec_rb_enc_interned_str_cstr(VALUE self, VALUE str, VALUE enc) {
-  rb_encoding *e = rb_to_encoding(enc);
+  rb_encoding *e = NIL_P(enc) ? 0 : rb_to_encoding(enc);
   return rb_enc_interned_str_cstr(RSTRING_PTR(str), e);
 }
 
diff --git a/spec/ruby/optional/capi/string_spec.rb b/spec/ruby/optional/capi/string_spec.rb
index d9c20cf176..378bf7323f 100644
--- a/spec/ruby/optional/capi/string_spec.rb
+++ b/spec/ruby/optional/capi/string_spec.rb
@@ -1236,6 +1236,14 @@ end
     it "returns the same string as String#-@" do
       @s.rb_enc_interned_str_cstr("hello", Encoding::UTF_8).should.equal?(-"hello")
     end
+
+    ruby_bug "#20322", ""..."3.4" do
+      it "uses the default encoding if encoding is null" do
+        str = "hello"
+        val = @s.rb_enc_interned_str_cstr(str, nil)
+        val.encoding.should == Encoding::ASCII_8BIT
+      end
+    end
   end
 
   describe "rb_str_to_interned_str" do