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 8b75dae3..68ccacfb 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/controller/TranscriptionBlockControllerTest.java
@@ -199,6 +199,20 @@ class TranscriptionBlockControllerTest {
                 .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void createBlock_returns400_whenMentionedPersonPersonIdIsNull() throws Exception {
+        when(userService.findByEmail(any())).thenReturn(mockUser());
+        String body = "{\"pageNumber\":1,\"x\":0.1,\"y\":0.2,\"width\":0.3,\"height\":0.4,\"text\":\"x\","
+                + "\"mentionedPersons\":[{\"personId\":null,\"displayName\":\"Auguste Raddatz\"}]}";
+
+        mockMvc.perform(post(URL_BASE)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("VALIDATION_ERROR"));
+    }
+
     // ─── PUT /api/documents/{id}/transcription-blocks/{blockId} ─────────────
 
     @Test