diff --git a/src/mdformat/renderer/_context.py b/src/mdformat/renderer/_context.py
index 59ce666..a4f623e 100644
--- a/src/mdformat/renderer/_context.py
+++ b/src/mdformat/renderer/_context.py
@@ -151,8 +151,8 @@ def fence(node: RenderTreeNode, context: RenderContext) -> str:
     fence_char = "~" if "`" in info_str else "`"
 
     # Format the code block using enabled codeformatter funcs
-    if lang in context.options.get("codeformatters", {}):
-        fmt_func = context.options["codeformatters"][lang]
+    fmt_func = context.options.get("codeformatters", {}).get(lang)
+    if fmt_func:
         try:
             code_block = fmt_func(code_block, info_str)
         except Exception:
@@ -167,6 +167,9 @@ def fence(node: RenderTreeNode, context: RenderContext) -> str:
             if filename:
                 warn_msg += f". Filename: {filename}"
             LOGGER.warning(warn_msg)
+        else:
+            if code_block and code_block[-1] != "\n":
+                code_block += "\n"
 
     # The code block must not include as long or longer sequence of `fence_char`s
     # as the fence string itself
diff --git a/tests/test_plugins.py b/tests/test_plugins.py
index 27c4e81..b7c0f8e 100644
--- a/tests/test_plugins.py
+++ b/tests/test_plugins.py
@@ -12,12 +12,11 @@
 from mdformat.renderer import MDRenderer, RenderContext, RenderTreeNode
 
 
-def example_formatter(code, info):
-    return "dummy\n"
-
-
 def test_code_formatter(monkeypatch):
-    monkeypatch.setitem(CODEFORMATTERS, "lang", example_formatter)
+    def fmt_func(code, info):
+        return "dummy\n"
+
+    monkeypatch.setitem(CODEFORMATTERS, "lang", fmt_func)
     text = mdformat.text(
         dedent(
             """\
@@ -37,6 +36,82 @@ def test_code_formatter(monkeypatch):
     )
 
 
+def test_code_formatter__empty_str(monkeypatch):
+    def fmt_func(code, info):
+        return ""
+
+    monkeypatch.setitem(CODEFORMATTERS, "lang", fmt_func)
+    text = mdformat.text(
+        dedent(
+            """\
+    ~~~lang
+    aag
+    gw
+    ~~~
+    """
+        ),
+        codeformatters={"lang"},
+    )
+    assert text == dedent(
+        """\
+    ```lang
+    ```
+    """
+    )
+
+
+def test_code_formatter__no_end_newline(monkeypatch):
+    def fmt_func(code, info):
+        return "dummy\ndum"
+
+    monkeypatch.setitem(CODEFORMATTERS, "lang", fmt_func)
+    text = mdformat.text(
+        dedent(
+            """\
+    ```lang
+    ```
+    """
+        ),
+        codeformatters={"lang"},
+    )
+    assert text == dedent(
+        """\
+    ```lang
+    dummy
+    dum
+    ```
+    """
+    )
+
+
+def test_code_formatter__interface(monkeypatch):
+    def fmt_func(code, info):
+        return info + code * 2
+
+    monkeypatch.setitem(CODEFORMATTERS, "lang", fmt_func)
+    text = mdformat.text(
+        dedent(
+            """\
+    ```    lang  long
+    multi
+    mul
+    ```
+    """
+        ),
+        codeformatters={"lang"},
+    )
+    assert text == dedent(
+        """\
+    ```lang  long
+    lang  longmulti
+    mul
+    multi
+    mul
+    ```
+    """
+    )
+
+
 class TextEditorPlugin:
     """A plugin that makes all text the same."""