diff --git a/mmcv/ops/multi_scale_deform_attn.py b/mmcv/ops/multi_scale_deform_attn.py
index 92759a1d8f..793e350c25 100644
--- a/mmcv/ops/multi_scale_deform_attn.py
+++ b/mmcv/ops/multi_scale_deform_attn.py
@@ -332,7 +332,10 @@ def forward(self,
 
         bs, num_query, _ = query.shape
         bs, num_value, _ = value.shape
-        assert (spatial_shapes[:, 0] * spatial_shapes[:, 1]).sum() == num_value
+        if (spatial_shapes[:, 0] * spatial_shapes[:, 1]).sum() != num_value:
+            raise ValueError('The sequence length of `value` must be equal to'
+                             'size of the flattened features maps summed over'
+                             'all levels.')
 
         value = self.value_proj(value)
         if key_padding_mask is not None: