diff --git a/src/errors.h b/src/errors.h
index 6596199..c76afbc 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -256,4 +256,6 @@
 	INIT(= N_("E1115: assert_fails() fourth argument must be a number"));
 EXTERN char e_assert_fails_fifth_argument[]
 	INIT(= N_("E1116: assert_fails() fifth argument must be a string"));
+EXTERN char e_cannot_use_bang_with_nested_def[]
+	INIT(= N_("E1117: Cannot use ! with nested :def"));
 #endif
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 4e79c89..e0b43ae 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -134,6 +134,23 @@
   CheckDefFailure(['func Nested()', 'endfunc'], 'E1086:')
   CheckDefFailure(['def s:Nested()', 'enddef'], 'E1075:')
   CheckDefFailure(['def b:Nested()', 'enddef'], 'E1075:')
+
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def Inner()',
+        '  enddef',
+        'enddef'], 'E1073:')
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def! Inner()',
+        '  enddef',
+        'enddef'], 'E1117:')
 enddef
 
 func Test_call_default_args_from_func()
diff --git a/src/version.c b/src/version.c
index 7aa35c2..5386822 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1657,
+/**/
     1656,
 /**/
     1655,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 7199e85..43d994e 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4320,6 +4320,12 @@
     ufunc_T	*ufunc;
     int		r;
 
+    if (*name_start == '!')
+    {
+	emsg(_(e_cannot_use_bang_with_nested_def));
+	return NULL;
+    }
+
     // Only g:Func() can use a namespace.
     if (name_start[1] == ':' && !is_global)
     {