diff --git a/internal/common/errors.go b/internal/common/errors.go
index b7bfdbb..f93997b 100644
--- a/internal/common/errors.go
+++ b/internal/common/errors.go
@@ -17,10 +17,15 @@ func Error(status int, title string, detail string) ProblemJSONError {
 func CustomErrorHandler(ctx fiber.Ctx, err error) error {
 	var problemJSON *ProblemJSONError
 
-	// Retrieve the custom status code if it's a fiber.*Error
-	var e *fiber.Error
-	if errors.Is(err, e) {
-		problemJSON = &ProblemJSONError{Status: e.Code, Title: e.Message}
+	// Retrieve the custom status code if it's a fiber.Error.
+	fiberError, ok := errors.AsType[*fiber.Error](err)
+	if ok {
+		problemJSON = &ProblemJSONError{
+			Code:   "",
+			Title:  fiberError.Message,
+			Detail: "",
+			Status: fiberError.Code,
+		}
 	}
 
 	if problemJSON == nil {
diff --git a/main_test.go b/main_test.go
index 4d049b0..f5dad04 100644
--- a/main_test.go
+++ b/main_test.go
@@ -30,6 +30,7 @@ type TestCase struct {
 	expectedCode        int
 	expectedBody        string
 	expectedContentType string
+	expectedHeaders     map[string]string
 	validateFunc        func(t *testing.T, response map[string]any)
 }
 
@@ -107,6 +108,9 @@ func runTestCases(t *testing.T, tests []TestCase) {
 			}
 
 			assert.Equal(t, test.expectedContentType, res.Header.Get("Content-Type"))
+			for name, expectedValue := range test.expectedHeaders {
+				assert.Equal(t, expectedValue, res.Header.Get(name))
+			}
 		})
 	}
 }
@@ -118,7 +122,7 @@ func TestApi(t *testing.T) {
 			query:       "GET /v1/i-dont-exist",
 
 			expectedCode:        404,
-			expectedBody:        `{"title":"Not Found","detail":"Not Found","status":404}`,
+			expectedBody:        `{"title":"Not Found","status":404}`,
 			expectedContentType: "application/problem+json",
 		},
 	}
@@ -131,6 +135,16 @@ func TestValidateEndpoint(t *testing.T) {
 	invalidYml := loadTestdata(t, "invalid.publiccode.yml")
 
 	tests := []TestCase{
+		{
+			description:         "validate: unsupported method",
+			query:               "GET /v1/validate",
+			expectedCode:        405,
+			expectedBody:        `{"title":"Method Not Allowed","status":405}`,
+			expectedContentType: "application/problem+json",
+			expectedHeaders: map[string]string{
+				"Allow": "POST, QUERY",
+			},
+		},
 		{
 			description:         "validate: empty body",
 			query:               "QUERY /v1/validate",
diff --git a/publiccode-validator-api.oas.yaml b/publiccode-validator-api.oas.yaml
index c4e3cb4..772d6de 100644
--- a/publiccode-validator-api.oas.yaml
+++ b/publiccode-validator-api.oas.yaml
@@ -105,6 +105,21 @@ paths:
                 title: empty body
                 detail: need a body to validate
                 status: 400
+        "405":
+          description: Method not allowed
+          headers:
+            Allow:
+              description: Supported HTTP methods for this endpoint
+              schema:
+                type: string
+              example: POST, QUERY
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemJSON"
+              example:
+                title: Method Not Allowed
+                status: 405
         "404":
           description: Not found
           content:
@@ -113,7 +128,6 @@ paths:
                 $ref: "#/components/schemas/ProblemJSON"
               example:
                 title: Not Found
-                detail: Not Found
                 status: 404
 
 components: