diff --git a/src/proto/vim9class.pro b/src/proto/vim9class.pro
index edbddd2..66c6107 100644
--- a/src/proto/vim9class.pro
+++ b/src/proto/vim9class.pro
@@ -6,7 +6,7 @@
 void ex_type(exarg_T *eap);
 int class_object_index(char_u **arg, typval_T *rettv, evalarg_T *evalarg, int verbose);
 ufunc_T *find_class_func(char_u **arg);
-int class_member_exists(char_u *name, class_T **cl_ret, int *idx_ret, cctx_T *cctx);
+int class_member_index(char_u *name, size_t len, class_T **cl_ret, cctx_T *cctx);
 void copy_object(typval_T *from, typval_T *to);
 void object_unref(object_T *obj);
 void copy_class(typval_T *from, typval_T *to);
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index 9dd6f83..8cb9113 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -367,7 +367,8 @@
   v9.CheckScriptFailure(lines, 'E1041:')
 enddef
 
-def Test_class_member_access()
+def Test_class_member()
+  # check access rules
   var lines =<< trim END
       vim9script
       class TextPos
@@ -401,6 +402,38 @@
       assert_equal(17, TextPos.anybody)
   END
   v9.CheckScriptSuccess(lines)
+
+  # check shadowing
+  lines =<< trim END
+      vim9script
+
+      class Some
+        static count = 0
+        def Method(count: number)
+          echo count
+        enddef
+      endclass
+
+      var s = Some.new()
+      s.Method(7)
+  END
+  v9.CheckScriptFailure(lines, 'E1340: Argument already declared in the class: count')
+
+  lines =<< trim END
+      vim9script
+
+      class Some
+        static count = 0
+        def Method(arg: number)
+          var count = 3
+          echo arg count
+        enddef
+      endclass
+
+      var s = Some.new()
+      s.Method(7)
+  END
+  v9.CheckScriptFailure(lines, 'E1341: Variable already declared in the class: count')
 enddef
 
 def Test_class_function()
diff --git a/src/version.c b/src/version.c
index 389209a..dc320be 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1127,
+/**/
     1126,
 /**/
     1125,
diff --git a/src/vim9class.c b/src/vim9class.c
index b05dea8..8e05a17 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -358,17 +358,19 @@
 	    ufunc_T *uf = define_function(&ea, NULL, &lines_to_free, TRUE);
 	    ga_clear_strings(&lines_to_free);
 
-	    // TODO: how about errors?
-	    int is_new = STRNCMP(uf->uf_name, "new", 3) == 0;
-	    garray_T *fgap = has_static || is_new
-					       ? &classfunctions : &objmethods;
-	    if (uf != NULL && ga_grow(fgap, 1) == OK)
+	    if (uf != NULL)
 	    {
-		if (is_new)
-		    uf->uf_flags |= FC_NEW;
+		int is_new = STRNCMP(uf->uf_name, "new", 3) == 0;
+		garray_T *fgap = has_static || is_new
+					       ? &classfunctions : &objmethods;
+		if (ga_grow(fgap, 1) == OK)
+		{
+		    if (is_new)
+			uf->uf_flags |= FC_NEW;
 
-		((ufunc_T **)fgap->ga_data)[fgap->ga_len] = uf;
-		++fgap->ga_len;
+		    ((ufunc_T **)fgap->ga_data)[fgap->ga_len] = uf;
+		    ++fgap->ga_len;
+		}
 	    }
 	}
 
@@ -863,32 +865,30 @@
 }
 
 /*
- * If "cctx->ctx_ufunc" indicates we are in a class, check if "name" is a class
- * member.  If it is then return TRUE and set "cl_ret" and "idx_ret".
+ * If "name[len]" is a class member in cctx->ctx_ufunc->uf_class return the
+ * index in class.class_class_members[].
+ * If "cl_ret" is not NULL set it to the class.
+ * Otherwise return -1;
  */
     int
-class_member_exists(
-	char_u	*name,
-	class_T	**cl_ret,
-	int	*idx_ret,
-	cctx_T	*cctx)
+class_member_index(char_u *name, size_t len, class_T **cl_ret, cctx_T *cctx)
 {
-    if (cctx->ctx_ufunc == NULL || cctx->ctx_ufunc->uf_class == NULL)
-	return FALSE;
+    if (cctx == NULL || cctx->ctx_ufunc == NULL
+					  || cctx->ctx_ufunc->uf_class == NULL)
+	return -1;
     class_T *cl = cctx->ctx_ufunc->uf_class;
 
-    for (int idx = 0; idx < cl->class_class_member_count; ++idx)
+    for (int i = 0; i < cl->class_class_member_count; ++i)
     {
-	ocmember_T *m = &cl->class_class_members[idx];
-	if (STRCMP(m->ocm_name, name) == 0)
+	ocmember_T *m = &cl->class_class_members[i];
+	if (STRNCMP(name, m->ocm_name, len) == 0 && m->ocm_name[len] == NUL)
 	{
-	    *cl_ret = cl;
-	    *idx_ret = idx;
-	    return TRUE;
+	    if (cl_ret != NULL)
+		*cl_ret = cl;
+	    return i;
 	}
     }
-
-    return FALSE;
+    return -1;
 }
 
 /*
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 0d980ba..0e6c32c 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -302,28 +302,6 @@
 }
 
 /*
- * If "name" is a class member in cctx->ctx_ufunc->uf_class return the index in
- * class.class_class_members[].
- * Otherwise return -1;
- */
-    static int
-class_member_index(char_u *name, size_t len, cctx_T *cctx)
-{
-    if (cctx == NULL || cctx->ctx_ufunc == NULL
-					  || cctx->ctx_ufunc->uf_class == NULL)
-	return -1;
-    class_T *cl = cctx->ctx_ufunc->uf_class;
-    for (int i = 0; i < cl->class_class_member_count; ++i)
-    {
-	ocmember_T *m = &cl->class_class_members[i];
-	if (STRNCMP(name, m->ocm_name, len) == 0
-		&& m->ocm_name[len] == NUL)
-	    return i;
-    }
-    return -1;
-}
-
-/*
  * Return TRUE if "name" is a local variable, argument, script variable or
  * imported.
  */
@@ -338,7 +316,7 @@
 			&& (cctx->ctx_ufunc->uf_flags & FC_OBJECT)
 			&& STRNCMP(name, "this", 4) == 0)))
 	    || script_var_exists(name, len, cctx, NULL) == OK
-	    || class_member_index(name, len, cctx) >= 0
+	    || class_member_index(name, len, NULL, cctx) >= 0
 	    || find_imported(name, len, FALSE) != NULL;
 }
 
@@ -376,9 +354,6 @@
     if (len == 1 && *p == '_')
 	return OK;
 
-    if (class_member_index(p, len, cctx) >= 0)
-	return OK;
-
     if (script_var_exists(p, len, cctx, cstack) == OK)
     {
 	if (is_arg)
@@ -388,6 +363,15 @@
 	return FAIL;
     }
 
+    if (class_member_index(p, len, NULL, cctx) >= 0)
+    {
+	if (is_arg)
+	    semsg(_(e_argument_already_declared_in_class_str), p);
+	else
+	    semsg(_(e_variable_already_declared_in_class_str), p);
+	return FAIL;
+    }
+
     p[len] = NUL;
     if ((cctx != NULL
 		&& (lookup_local(p, len, NULL, cctx) == OK
@@ -1592,8 +1576,14 @@
 		}
 	    }
 	    else if ((lhs->lhs_classmember_idx = class_member_index(
-				       var_start, lhs->lhs_varlen, cctx)) >= 0)
+				 var_start, lhs->lhs_varlen, NULL, cctx)) >= 0)
 	    {
+		if (is_decl)
+		{
+		    semsg(_(e_variable_already_declared_in_class_str),
+								lhs->lhs_name);
+		    return FAIL;
+		}
 		lhs->lhs_dest = dest_class_member;
 		lhs->lhs_class = cctx->ctx_ufunc->uf_class;
 	    }
@@ -2264,7 +2254,7 @@
     CLEAR_FIELD(lhs);
     long	start_lnum = SOURCING_LNUM;
 
-    int		has_arg_is_set_prefix = STRNCMP(arg, "ifargisset ", 11) == 0;
+    int	has_arg_is_set_prefix = STRNCMP(arg, "ifargisset ", 11) == 0;
     if (has_arg_is_set_prefix)
     {
 	arg += 11;
diff --git a/src/vim9expr.c b/src/vim9expr.c
index 463e79c..4a3527f 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -603,7 +603,7 @@
 		else
 		    gen_load = TRUE;
 	    }
-	    else if (class_member_exists(name, &cl, &idx, cctx))
+	    else if ((idx = class_member_index(*arg, len, &cl, cctx)) >= 0)
 	    {
 		res = generate_CLASSMEMBER(cctx, TRUE, cl, idx);
 	    }