diff --git a/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java b/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java
index 0d39884a..bed7ff8f 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java
@@ -266,6 +266,19 @@ class TranscriptionBlockControllerTest {
                 .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void updateBlock_returns400_whenMentionedPersonPersonIdIsNull() throws Exception {
+        when(userService.findByEmail(any())).thenReturn(mockUser());
+        String body = "{\"text\":\"x\",\"mentionedPersons\":[{\"personId\":null,\"displayName\":\"Auguste Raddatz\"}]}";
+
+        mockMvc.perform(put(URL_BLOCK)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
+    }
+
     @Test
     @WithMockUser(authorities = "WRITE_ALL")
     void updateBlock_returns404_whenBlockDoesNotExist() throws Exception {