diff --git a/integ_tests/test_sharing.py b/integ_tests/test_sharing.py
index b7fc796e..f1d53c58 100644
--- a/integ_tests/test_sharing.py
+++ b/integ_tests/test_sharing.py
@@ -246,3 +246,36 @@ def test_edit_share_by_map(page: Page, radicale_server: str) -> None:
     page.click('tr:not(.hidden) button[data-name="edit"]')
     expect(page.locator('input[data-name="enabled"]')).not_to_be_checked()
     page.click('#newshare button[data-name="cancel"]')
+
+
+def test_share_by_map_validation(page: Page, radicale_server: str) -> None:
+    login(page, radicale_server)
+    create_collection(page, radicale_server)
+    page.hover("article:not(.hidden)")
+    page.click('article:not(.hidden) a[data-name="share"]', force=True, strict=True)
+
+    page.click('button[data-name="sharebymap"]')
+
+    # Try empty user
+    page.locator('input[data-name="shareuser"]').fill("")
+    page.locator('input[data-name="sharehref"]').fill("1234")
+    page.click('#newshare button[data-name="submit"]')
+    expect(page.locator('#newshare [data-name="error"]:not(.hidden)')).to_contain_text(
+        "Share User is empty"
+    )
+
+    # Try logged in user
+    page.locator('input[data-name="shareuser"]').fill("admin")
+    page.click('#newshare button[data-name="submit"]')
+    expect(page.locator('#newshare [data-name="error"]:not(.hidden)')).to_contain_text(
+        "Share User cannot be admin"
+    )
+
+    # Valid user
+    page.locator('input[data-name="shareuser"]').fill("max")
+    page.click('#newshare button[data-name="submit"]')
+
+    # Verify success
+    expect(
+        page.locator("tr[data-name='sharemaprowtemplate']:not(.hidden)")
+    ).to_have_count(1)
diff --git a/radicale/web/internal_data/js/scenes/CreateEditShareScene.js b/radicale/web/internal_data/js/scenes/CreateEditShareScene.js
index cb6d83d2..24b1c5a2 100644
--- a/radicale/web/internal_data/js/scenes/CreateEditShareScene.js
+++ b/radicale/web/internal_data/js/scenes/CreateEditShareScene.js
@@ -22,7 +22,7 @@
 import { Share, add_share_by_map, add_share_by_token, get_property_key, update_share_by_map, update_share_by_token } from "../api/sharing.js";
 import { CollectionType } from "../models/collection.js";
 import { ErrorHandler } from "../utils/error.js";
-import { FormValidator, validate_href, validate_non_empty } from "../utils/form_validator.js";
+import { FormValidator, validate_href, validate_not_empty_or_equals } from "../utils/form_validator.js";
 import { onCleanHREFinput, random_uuid } from "../utils/misc.js";
 import { Scene, pop_scene, scene_stack } from "./scene_manager.js";
 
@@ -64,7 +64,7 @@ export class CreateEditShareScene {
         let errorHandler = new ErrorHandler(error_form);
         let map_validator = new FormValidator(errorHandler);
 
-        map_validator.addValidator(shareuser_input, validate_non_empty(shareuser_input, "Share User"));
+        map_validator.addValidator(shareuser_input, validate_not_empty_or_equals(shareuser_input, user, "Share User"));
         map_validator.addValidator(sharehref_input, validate_href(sharehref_input, "Share Href"));
 
         sharehref_input.addEventListener("input", onCleanHREFinput);
diff --git a/radicale/web/internal_data/js/utils/form_validator.js b/radicale/web/internal_data/js/utils/form_validator.js
index 21cd0826..2cd53a76 100644
--- a/radicale/web/internal_data/js/utils/form_validator.js
+++ b/radicale/web/internal_data/js/utils/form_validator.js
@@ -76,6 +76,27 @@ export function validate_non_empty(input, field_name) {
         return field_name + " is empty";
     };
 }
+
+/**
+ * Validates that the input is not empty and not equal to a target string.
+ * @param {HTMLInputElement} input
+ * @param {string} target
+ * @param {string} field_name
+ * @returns {function(): ?string}
+ */
+export function validate_not_empty_or_equals(input, target, field_name) {
+    return () => {
+        let value = input.value.trim();
+        if (!value) {
+            return field_name + " is empty";
+        }
+        if (value === target) {
+            return field_name + " cannot be " + target;
+        }
+        return null;
+    };
+}
+
 /**
  * Validates that the input is a valid HREF.
  * @param {HTMLInputElement} input