diff --git a/src/transformers/models/conditional_detr/image_processing_conditional_detr.py b/src/transformers/models/conditional_detr/image_processing_conditional_detr.py
index 2fe33db810..70e12b0ddc 100644
--- a/src/transformers/models/conditional_detr/image_processing_conditional_detr.py
+++ b/src/transformers/models/conditional_detr/image_processing_conditional_detr.py
@@ -1414,13 +1414,14 @@ class ConditionalDetrImageProcessor(BaseImageProcessor):
         boxes = torch.gather(boxes, 1, topk_boxes.unsqueeze(-1).repeat(1, 1, 4))
 
         # and from relative [0, 1] to absolute [0, height] coordinates
-        if isinstance(target_sizes, List):
-            img_h = torch.Tensor([i[0] for i in target_sizes])
-            img_w = torch.Tensor([i[1] for i in target_sizes])
-        else:
-            img_h, img_w = target_sizes.unbind(1)
-        scale_fct = torch.stack([img_w, img_h, img_w, img_h], dim=1).to(boxes.device)
-        boxes = boxes * scale_fct[:, None, :]
+        if target_sizes is not None:
+            if isinstance(target_sizes, List):
+                img_h = torch.Tensor([i[0] for i in target_sizes])
+                img_w = torch.Tensor([i[1] for i in target_sizes])
+            else:
+                img_h, img_w = target_sizes.unbind(1)
+            scale_fct = torch.stack([img_w, img_h, img_w, img_h], dim=1).to(boxes.device)
+            boxes = boxes * scale_fct[:, None, :]
 
         results = []
         for s, l, b in zip(scores, labels, boxes):
diff --git a/src/transformers/models/deformable_detr/image_processing_deformable_detr.py b/src/transformers/models/deformable_detr/image_processing_deformable_detr.py
index 8c40d20c81..5261170062 100644
--- a/src/transformers/models/deformable_detr/image_processing_deformable_detr.py
+++ b/src/transformers/models/deformable_detr/image_processing_deformable_detr.py
@@ -1411,13 +1411,14 @@ class DeformableDetrImageProcessor(BaseImageProcessor):
         boxes = torch.gather(boxes, 1, topk_boxes.unsqueeze(-1).repeat(1, 1, 4))
 
         # and from relative [0, 1] to absolute [0, height] coordinates
-        if isinstance(target_sizes, List):
-            img_h = torch.Tensor([i[0] for i in target_sizes])
-            img_w = torch.Tensor([i[1] for i in target_sizes])
-        else:
-            img_h, img_w = target_sizes.unbind(1)
-        scale_fct = torch.stack([img_w, img_h, img_w, img_h], dim=1).to(boxes.device)
-        boxes = boxes * scale_fct[:, None, :]
+        if target_sizes is not None:
+            if isinstance(target_sizes, List):
+                img_h = torch.Tensor([i[0] for i in target_sizes])
+                img_w = torch.Tensor([i[1] for i in target_sizes])
+            else:
+                img_h, img_w = target_sizes.unbind(1)
+            scale_fct = torch.stack([img_w, img_h, img_w, img_h], dim=1).to(boxes.device)
+            boxes = boxes * scale_fct[:, None, :]
 
         results = []
         for s, l, b in zip(scores, labels, boxes):