1 files changed, 20 insertions, 0 deletions
diff --git a/test/-ext-/string/test_cstr.rb b/test/-ext-/string/test_cstr.rb
index 6f2fdef3d4..272e090955 100644
--- a/test/-ext-/string/test_cstr.rb
+++ b/test/-ext-/string/test_cstr.rb
@@ -86,6 +86,26 @@ class Test_StringCStr < Test::Unit::TestCase
     }
   end
 
+  def test_embedded_from_heap
+    gh821 = "[GH-821]"
+    embedded_string = "abcdefghi"
+    string = embedded_string.gsub("efg", "123")
+    {}[string] = 1
+    non_terminated = "#{string}#{nil}"
+    assert_nil(Bug::String.cstr_term_char(non_terminated), gh821)
+
+    result = {}
+    WCHARS.map do |enc|
+      embedded_string = "ab".encode(enc)
+      string = embedded_string.gsub("b".encode(enc), "1".encode(enc))
+      {}[string] = 1
+      non_terminated = "#{string}#{nil}"
+      c = Bug::String.cstr_term_char(non_terminated)
+      result[enc] = c if c
+    end
+    assert_empty(result, gh821)
+  end
+
   def assert_wchars_term_char(str)
     result = {}
     WCHARS.map do |enc|