diff --git a/sbi/user_input/user_input_checks.py b/sbi/user_input/user_input_checks.py
index 4cf2a2ab4..76662171d 100644
--- a/sbi/user_input/user_input_checks.py
+++ b/sbi/user_input/user_input_checks.py
@@ -122,7 +122,7 @@ def maybe_wrap_prior_as_pytorch(prior) -> Tuple[Distribution, bool]:
         is_prior_numpy = True
     else:
         raise TypeError(
-            "Prior must return torch.Tensor or ndarray, but returns {type(theta)}"
+            f"Prior must return torch.Tensor or ndarray, but returns {type(theta)}"
         )
 
     return cast(Distribution, prior), is_prior_numpy