diff --git a/src/errors.h b/src/errors.h
index 1e59597..ad36e33 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3616,8 +3616,10 @@
 	INIT(= N_("E1428: Duplicate enum value: %s"));
 EXTERN char e_class_can_only_be_used_in_script[]
 	INIT(= N_("E1429: Class can only be used in a script"));
+EXTERN char e_uninitialized_object_var_reference[]
+	INIT(= N_("E1430: Uninitialized object variable '%s' referenced"));
 #endif
-// E1429 - E1499 unused (reserved for Vim9 class support)
+// E1431 - E1499 unused (reserved for Vim9 class support)
 EXTERN char e_cannot_mix_positional_and_non_positional_str[]
 	INIT(= N_("E1500: Cannot mix positional and non-positional arguments: %s"));
 EXTERN char e_fmt_arg_nr_unused_str[]
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index 4a7962a..c7a0fbe 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -11723,4 +11723,120 @@
   v9.CheckSourceFailure(lines, 'E1326: Variable "NewCost" not found in object "Foo"')
 enddef
 
+" Test for referencing an object variable which is not yet initialized
+def Test_uninitialized_object_var()
+  var lines =<< trim END
+    vim9script
+    class Foo
+      const two: number = Foo.Two(this)
+      const one: number = 1
+
+      static def Two(that: Foo): number
+        return that.one + 2
+      enddef
+    endclass
+
+    echo Foo.Two(Foo.new())
+  END
+  v9.CheckSourceFailure(lines, "E1430: Uninitialized object variable 'one' referenced")
+
+  lines =<< trim END
+    vim9script
+    class Foo
+      const one: number = Foo.One(this)
+
+      static def One(that: Foo): number
+        return 1
+      enddef
+    endclass
+
+    assert_equal(1, Foo.One(Foo.new()))
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    class Foo
+      const one: number = 1
+      const two: number = Foo.Two(this)
+
+      static def Two(that: Foo): number
+        return that.one + 1
+      enddef
+    endclass
+
+    assert_equal(2, Foo.Two(Foo.new()))
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    class Foo
+      const Id: func(any): any = ((_) => (v) => v)(this)
+
+      static def Id(that: Foo): func(any): any
+        return that.Id
+      enddef
+    endclass
+
+    assert_equal(5, Foo.Id(Foo.new())(5))
+    assert_equal(7, Foo.new().Id(7))
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    class Foo
+      const Id: func(any): any = ((that) => (_) => that)(this)
+
+      static def Id(that: Foo): func(any): any
+          return that.Id
+      enddef
+    endclass
+
+    const Id0: func(any): any = Foo.Id(Foo.new())
+    const Id1: func(any): any = Foo.new().Id
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    class Foo
+      const Id: any = Foo.Id(this)
+
+      static def Id(that: Foo): any
+          return that.Id
+      enddef
+    endclass
+
+    const Id2: any = Foo.Id(Foo.new())
+    const Id3: any = Foo.new().Id
+  END
+  v9.CheckSourceFailure(lines, "E1430: Uninitialized object variable 'Id' referenced")
+
+  lines =<< trim END
+    vim9script
+
+    class Foo
+      var x: string = ''
+      var Y: func(): string = () => this.x
+    endclass
+
+    var foo = Foo.new('ok')
+    assert_equal('ok', foo.Y())
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+
+    class Foo
+      var x: string = this.x
+    endclass
+
+    var foo = Foo.new('ok')
+  END
+  v9.CheckSourceFailure(lines, "E1430: Uninitialized object variable 'x' referenced")
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker
diff --git a/src/version.c b/src/version.c
index 1dbbca5..987aa9b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    988,
+/**/
     987,
 /**/
     986,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index de12d8e..dde95b5 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -4844,6 +4844,20 @@
 		int arg_set = tv->v_type != VAR_UNKNOWN
 				&& !(tv->v_type == VAR_SPECIAL
 					    && tv->vval.v_number == VVAL_NONE);
+
+		if (iptr->isn_type == ISN_JUMP_IF_ARG_NOT_SET && !arg_set)
+		{
+		    dfunc_T *df = ((dfunc_T *)def_functions.ga_data)
+							+ ectx->ec_dfunc_idx;
+		    ufunc_T *ufunc = df->df_ufunc;
+		    // jump_arg_off is negative for arguments
+		    size_t argidx = ufunc->uf_def_args.ga_len
+					+ iptr->isn_arg.jumparg.jump_arg_off
+					+ STACK_FRAME_SIZE;
+		    type_T *t = ufunc->uf_arg_types[argidx];
+		    tv->v_type = t->tt_type;
+		}
+
 		if (iptr->isn_type == ISN_JUMP_IF_ARG_SET ? arg_set : !arg_set)
 		    ectx->ec_iidx = iptr->isn_arg.jumparg.jump_where;
 		break;
@@ -5718,6 +5732,17 @@
 
 		    // The members are located right after the object struct.
 		    typval_T *mtv = ((typval_T *)(obj + 1)) + idx;
+		    if (mtv->v_type == VAR_UNKNOWN)
+		    {
+			// Referencing an object variable (without a type)
+			// which is not yet initialized.  So the type is not
+			// yet known.
+			ocmember_T *m = &obj->obj_class->class_obj_members[idx];
+			SOURCING_LNUM = iptr->isn_lnum;
+			semsg(_(e_uninitialized_object_var_reference),
+				m->ocm_name);
+			goto on_error;
+		    }
 		    copy_tv(mtv, tv);
 
 		    // Unreference the object after getting the member, it may