diff --git a/functions/is-problem-json-schema.js b/functions/is-problem-json-schema.js
index b42804f..4d71cd4 100644
--- a/functions/is-problem-json-schema.js
+++ b/functions/is-problem-json-schema.js
@@ -45,8 +45,33 @@ const assertProblemSchema = (schema) => {
   }
 };
 
+/*
+ * Merge list of schema definitions of type = 'object'.
+ * Return object will have a super set of attributes 'properties' and 'required'.
+ */
+const mergeObjectDefinitions = (allOfTypes) => {
+  if (allOfTypes.filter((item) => item.type !== 'object').length !== 0) {
+    throw "All schema definitions must be of type 'object'";
+  }
+
+  return allOfTypes.reduce((acc, item) => {
+    return {
+      type: 'object',
+      properties: { ...(acc.properties || {}), ...(item.properties || {}) },
+      required: [...(acc.required || []), ...(item.required || [])],
+    };
+  }, {});
+};
+
 const check = (schema) => {
-  const combinedSchemas = [...(schema.anyOf || []), ...(schema.oneOf || []), ...(schema.allOf || [])];
+  const combinedSchemas = [...(schema.anyOf || []), ...(schema.oneOf || [])];
+  if (schema.allOf) {
+    const mergedAllOf = mergeObjectDefinitions(schema.allOf);
+    if (mergedAllOf) {
+      combinedSchemas.push(mergedAllOf);
+    }
+  }
+
   if (combinedSchemas.length > 0) {
     combinedSchemas.forEach(check);
   } else {
diff --git a/tests/176-MUST-support-problem-JSON.test.ts b/tests/176-MUST-support-problem-JSON.test.ts
index 97f03f7..0384b03 100644
--- a/tests/176-MUST-support-problem-JSON.test.ts
+++ b/tests/176-MUST-support-problem-JSON.test.ts
@@ -44,4 +44,36 @@ describe('MUST support problem JSON [176]', () => {
       }),
     ]);
   });
+
+  test('Verify custom problem extending valid problem object does not cause error', async () => {
+    const openApi = await loadOpenApiSpec('base-openapi.yml');
+
+    // define custom problem extending problem json
+    openApi.components.schemas['CustomValidationProblem'] = {
+      allOf: [
+        { $ref: '#/components/schemas/Problem' },
+        {
+          type: 'object',
+          required: ['title', 'status', 'detail', 'validation_error'],
+          properties: {
+            validation_error: { type: 'string' },
+          },
+        },
+      ],
+    };
+
+    openApi.paths['/example'].get.responses['400'] = {
+      description: 'bad request',
+      content: {
+        'application/problem+json': {
+          schema: {
+            $ref: '#/components/schemas/CustomValidationProblem',
+          },
+        },
+      },
+    };
+
+    const result = await lint(openApi);
+    expect(result).toEqual([]);
+  });
 });