diff --git a/src/diffusers/models/unet_2d.py b/src/diffusers/models/unet_2d.py
index db6d3a5dce3f..38e26422e2a7 100644
--- a/src/diffusers/models/unet_2d.py
+++ b/src/diffusers/models/unet_2d.py
@@ -291,6 +291,8 @@ def forward(
 
             class_emb = self.class_embedding(class_labels).to(dtype=self.dtype)
             emb = emb + class_emb
+        elif self.class_embedding is None and class_labels is not None:
+            raise ValueError("class_embedding needs to be initialized in order to use class conditioning")
 
         # 2. pre-process
         skip_sample = sample