diff --git a/packages/core/src/browser/shell/shell-layout-restorer.ts b/packages/core/src/browser/shell/shell-layout-restorer.ts
index e7db537a0d0ea..8b7d42df347d5 100644
--- a/packages/core/src/browser/shell/shell-layout-restorer.ts
+++ b/packages/core/src/browser/shell/shell-layout-restorer.ts
@@ -278,12 +278,12 @@ export class ShellLayoutRestorer implements CommandContribution {
                     });
                 }
                 return widgets;
-            } else if (isObject<Record<string, WidgetDescription>>(value) && !Array.isArray(value)) {
+            } else if (isObject(value) && !Array.isArray(value)) {
                 const copy: Record<string, unknown> = {};
                 for (const p in value) {
                     if (this.isWidgetProperty(p)) {
                         parseContext.push(async context => {
-                            copy[p] = await this.convertToWidget(value[p], context);
+                            copy[p] = await this.convertToWidget(value[p] as WidgetDescription, context);
                         });
                     } else {
                         copy[p] = value[p];
diff --git a/packages/core/src/common/types.ts b/packages/core/src/common/types.ts
index fff7231299fce..6fc5a93d326fe 100644
--- a/packages/core/src/common/types.ts
+++ b/packages/core/src/common/types.ts
@@ -17,6 +17,8 @@
 export { ArrayUtils } from './array-utils';
 export { Prioritizeable } from './prioritizeable';
 
+type UnknownObject<T extends object> = Record<string | number | symbol, unknown> & { [K in keyof T]: unknown };
+
 export type Deferred<T> = { [P in keyof T]: Promise<T[P]> };
 export type MaybeArray<T> = T | T[];
 export type MaybeNull<T> = { [P in keyof T]: T[P] | null };
@@ -54,7 +56,7 @@ export function isFunction<T extends (...args: unknown[]) => unknown>(value: unk
     return typeof value === 'function';
 }
 
-export function isObject<T = Record<string | number | symbol, unknown>>(value: unknown): value is T {
+export function isObject<T extends object>(value: unknown): value is UnknownObject<T> {
     // eslint-disable-next-line no-null/no-null
     return typeof value === 'object' && value !== null;
 }
diff --git a/packages/markers/src/common/marker.ts b/packages/markers/src/common/marker.ts
index 55e3c2e9bdaa6..4a5bf25f717bc 100644
--- a/packages/markers/src/common/marker.ts
+++ b/packages/markers/src/common/marker.ts
@@ -14,6 +14,8 @@
 // SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
 // *****************************************************************************
 
+import { isObject, isString } from '@theia/core/lib/common/types';
+
 /*
 * A marker represents meta information for a given uri
 */
@@ -37,3 +39,15 @@ export interface Marker<T> {
      */
     data: T;
 }
+export namespace Marker {
+    export function is(value: unknown): value is Marker<object>;
+    export function is<T>(value: unknown, subTypeCheck: (value: unknown) => value is T): value is Marker<T>;
+    export function is(value: unknown, subTypeCheck?: (value: unknown) => boolean): boolean {
+        subTypeCheck ??= isObject;
+        return isObject<Marker<object>>(value)
+            && !Array.isArray(value)
+            && subTypeCheck(value.data)
+            && isString(value.uri)
+            && isString(value.owner);
+    }
+}
diff --git a/packages/markers/src/common/problem-marker.ts b/packages/markers/src/common/problem-marker.ts
index 1c0493dbffb61..a9e56a4efbecb 100644
--- a/packages/markers/src/common/problem-marker.ts
+++ b/packages/markers/src/common/problem-marker.ts
@@ -24,7 +24,7 @@ export interface ProblemMarker extends Marker<Diagnostic> {
 }
 
 export namespace ProblemMarker {
-    export function is(node: Marker<object>): node is ProblemMarker {
-        return 'kind' in node && node.kind === PROBLEM_KIND;
+    export function is(node: unknown): node is ProblemMarker {
+        return Marker.is(node) && node.kind === PROBLEM_KIND;
     }
 }
diff --git a/packages/plugin-ext/src/plugin/types-impl.ts b/packages/plugin-ext/src/plugin/types-impl.ts
index d3e50b97221fd..ea589b4d176d7 100644
--- a/packages/plugin-ext/src/plugin/types-impl.ts
+++ b/packages/plugin-ext/src/plugin/types-impl.ts
@@ -404,10 +404,13 @@ export class Position {
         return result!;
     }
 
-    static isPosition(other: {}): other is Position {
+    static isPosition(other: unknown): other is Position {
         if (!other) {
             return false;
         }
+        if (typeof other !== 'object' || Array.isArray(other)) {
+            return false;
+        }
         if (other instanceof Position) {
             return true;
         }