diff --git a/sumdb/go.mod b/sumdb/go.mod
index a1a161f..83a9134 100644
--- a/sumdb/go.mod
+++ b/sumdb/go.mod
@@ -1,5 +1,5 @@
 module golang.org/x/exp/sumdb
 
-go 1.13
+go 1.12
 
 require golang.org/x/crypto v0.0.0-20190313024323-a1f597ede03a
diff --git a/sumdb/internal/sumweb/client.go b/sumdb/internal/sumweb/client.go
index 8306a67..364f48d 100644
--- a/sumdb/internal/sumweb/client.go
+++ b/sumdb/internal/sumweb/client.go
@@ -588,7 +588,7 @@
 		// We only save authenticated tiles to the on-disk cache,
 		// so the recreated prefix is equally authenticated.
 		full := tile
-		full.W = 1 << tile.H
+		full.W = 1 << uint(tile.H)
 		if tile != full {
 			data, err := c.client.ReadCache(c.tileCacheKey(full))
 			if err == nil {