--- a/src/structs.h
+++ b/src/structs.h
@@ -1500,6 +1500,7 @@ struct class_S
     // interfaces declared for the class
     int		class_interface_count;
     char_u	**class_interfaces;	// allocated array of names
+    class_T	**class_interfaces_cl;	// interfaces (counts as reference)
 
     // class members: "static varname"
     int		class_class_member_count;

--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -455,6 +455,24 @@ def Test_object_type()
       var o: One = Two.new()
   END
   v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected object<One> but got object<Two>')
+
+  lines =<< trim END
+      vim9script
+
+      interface One
+        def GetMember(): number
+      endinterface
+      class Two implements One
+        this.one = 1
+        def GetMember(): number
+          return this.one
+        enddef
+      endclass
+
+      var o: One = Two.new(5)
+      assert_equal(5, o.GetMember())
+  END
+  v9.CheckScriptSuccess(lines)
 enddef
 
 def Test_class_member()

--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@ static char *(features[]) =
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1184,
+/**/
     1183,
 /**/
     1182,

--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -582,9 +582,13 @@ early_ret:
     }
     VIM_CLEAR(extends);
 
+    class_T **intf_classes = NULL;
+
     // Check all "implements" entries are valid.
     if (success && ga_impl.ga_len > 0)
     {
+	intf_classes = ALLOC_CLEAR_MULT(class_T *, ga_impl.ga_len);
+
 	for (int i = 0; i < ga_impl.ga_len && success; ++i)
 	{
 	    char_u *impl = ((char_u **)ga_impl.ga_data)[i];
@@ -605,8 +609,11 @@ early_ret:
 		success = FALSE;
 	    }
 
+	    class_T *ifcl = tv.vval.v_class;
+	    intf_classes[i] = ifcl;
+	    ++ifcl->class_refcount;
+
 	    // check the members of the interface match the members of the class
-	    class_T *ifcl = tv.vval.v_class;
 	    for (int loop = 1; loop <= 2 && success; ++loop)
 	    {
 		// loop == 1: check class members
@@ -717,6 +724,9 @@ early_ret:
 		cl->class_interfaces[i] = ((char_u **)ga_impl.ga_data)[i];
 	    VIM_CLEAR(ga_impl.ga_data);
 	    ga_impl.ga_len = 0;
+
+	    cl->class_interfaces_cl = intf_classes;
+	    intf_classes = NULL;
 	}
 
 	// Add class and object members to "cl".
@@ -930,6 +940,18 @@ cleanup:
     {
 	vim_free(cl->class_name);
 	vim_free(cl->class_class_functions);
+	if (cl->class_interfaces != NULL)
+	{
+	    for (int i = 0; i < cl->class_interface_count; ++i)
+		vim_free(cl->class_interfaces[i]);
+	    vim_free(cl->class_interfaces);
+	}
+	if (cl->class_interfaces_cl != NULL)
+	{
+	    for (int i = 0; i < cl->class_interface_count; ++i)
+		class_unref(cl->class_interfaces_cl[i]);
+	    vim_free(cl->class_interfaces_cl);
+	}
 	vim_free(cl->class_obj_members);
 	vim_free(cl->class_obj_methods);
 	vim_free(cl);
@@ -937,6 +959,13 @@ cleanup:
 
     vim_free(extends);
     class_unref(extends_cl);
+
+    if (intf_classes != NULL)
+    {
+	for (int i = 0; i < ga_impl.ga_len; ++i)
+	    class_unref(intf_classes[i]);
+	vim_free(intf_classes);
+    }
     ga_clear_strings(&ga_impl);
 
     for (int round = 1; round <= 2; ++round)
@@ -1321,8 +1350,13 @@ class_unref(class_T *cl)
 	class_unref(cl->class_extends);
 
 	for (int i = 0; i < cl->class_interface_count; ++i)
+	{
 	    vim_free(((char_u **)cl->class_interfaces)[i]);
+	    if (cl->class_interfaces_cl[i] != NULL)
+		class_unref(cl->class_interfaces_cl[i]);
+	}
 	vim_free(cl->class_interfaces);
+	vim_free(cl->class_interfaces_cl);
 
 	for (int i = 0; i < cl->class_class_member_count; ++i)
 	{

--- a/src/vim9type.c
+++ b/src/vim9type.c
@@ -876,11 +876,21 @@ check_type_maybe(
 	}
 	else if (expected->tt_type == VAR_OBJECT)
 	{
+	    // check the class, base class or an implemented interface matches
 	    class_T *cl;
 	    for (cl = (class_T *)actual->tt_member; cl != NULL;
 							cl = cl->class_extends)
+	    {
 		if ((class_T *)expected->tt_member == cl)
 		    break;
+		int i;
+		for (i = cl->class_interface_count - 1; i >= 0; --i)
+		    if ((class_T *)expected->tt_member
+						 == cl->class_interfaces_cl[i])
+			break;
+		if (i >= 0)
+		    break;
+	    }
 	    if (cl == NULL)
 		ret = FAIL;
 	}