diff --git a/mmcv/ops/prroi_pool.py b/mmcv/ops/prroi_pool.py
index 63427a4ba34..2fe040bd88c 100644
--- a/mmcv/ops/prroi_pool.py
+++ b/mmcv/ops/prroi_pool.py
@@ -33,10 +33,9 @@ def forward(ctx,
                 output_size: Tuple,
                 spatial_scale: float = 1.0) -> torch.Tensor:
         if features.dtype() != torch.float or rois.dtype() != torch.float:
-            raise ValueError(
-                'Precise RoI Pooling only takes float input, got '
-                f'{features.dtype()} for features and'
-                f'{rois.dtype()} for rois.')
+            raise ValueError('Precise RoI Pooling only takes float input, got '
+                             f'{features.dtype()} for features and'
+                             f'{rois.dtype()} for rois.')
 
         pooled_height = int(output_size[0])
         pooled_width = int(output_size[1])