diff --git a/verify.go b/verify.go
index e71fc47..69305e6 100644
--- a/verify.go
+++ b/verify.go
@@ -32,7 +32,7 @@ it outputs nothing if the verification is successful.`,
 			}
 
 			if ok, err := evt.CheckSignature(); !ok {
-				ctx = lineProcessingError(ctx, "invalid signature: %s", err)
+				ctx = lineProcessingError(ctx, "invalid signature: %v", err)
 				continue
 			}
 		}