main.go | 3 +++

diff --git a/main.go b/main.go
index a05432858e5dfece95fd30e9a1bec8ea7a9b9b62..ce7a3d44ce88e35d64aaae2f0d19c7f2b9928435 100644
--- a/main.go
+++ b/main.go
@@ -157,6 +157,9 @@ 		}
 	}
 
 	bs = *blockSize * (1 << 10)
+	if bs > 1<<32 {
+		panic("blocksize exceeds 32-bits")
+	}
 	stdin := bufio.NewReaderSize(os.Stdin, LenSize+bs+poly1305.TagSize)
 
 	workers := make([]*Worker, *threads)