diff --git a/go/callgraph/vta/internal/trie/bits.go b/go/callgraph/vta/internal/trie/bits.go
index c343d29..f2fd0ba 100644
--- a/go/callgraph/vta/internal/trie/bits.go
+++ b/go/callgraph/vta/internal/trie/bits.go
@@ -46,7 +46,7 @@
 	if p == 0 {
 		return 0
 	}
-	return 1 << (bits.Len64(uint64(p)) - 1)
+	return bitpos(1) << uint(bits.Len64(uint64(p))-1) // uint conversion needed for go1.12
 }
 
 // zeroBit returns true if k has a 0 bit at position `b`.
diff --git a/go/callgraph/vta/internal/trie/bits_test.go b/go/callgraph/vta/internal/trie/bits_test.go
index f19acdf..07784cd 100644
--- a/go/callgraph/vta/internal/trie/bits_test.go
+++ b/go/callgraph/vta/internal/trie/bits_test.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.13
+// +build go1.13
+
 package trie
 
 import (
diff --git a/go/callgraph/vta/internal/trie/trie_test.go b/go/callgraph/vta/internal/trie/trie_test.go
index 68abb5b..c0651b0 100644
--- a/go/callgraph/vta/internal/trie/trie_test.go
+++ b/go/callgraph/vta/internal/trie/trie_test.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.13
+// +build go1.13
+
 package trie
 
 import (