diff --git a/src/cmd/go/internal/work/buildid.go b/src/cmd/go/internal/work/buildid.go
index 4e9189a36320ac..76335e9bb170b1 100644
--- a/src/cmd/go/internal/work/buildid.go
+++ b/src/cmd/go/internal/work/buildid.go
@@ -570,6 +570,8 @@ func showStdout(b *Builder, c *cache.Cache, actionID cache.ActionID, key string)
 			b.Showcmd("", "%s  # internal", joinUnambiguously(str.StringList("cat", c.OutputFile(stdoutEntry.OutputID))))
 		}
 		if !cfg.BuildN {
+			b.output.Lock()
+			defer b.output.Unlock()
 			b.Print(string(stdout))
 		}
 	}
@@ -578,6 +580,8 @@ func showStdout(b *Builder, c *cache.Cache, actionID cache.ActionID, key string)
 
 // flushOutput flushes the output being queued in a.
 func (b *Builder) flushOutput(a *Action) {
+	b.output.Lock()
+	defer b.output.Unlock()
 	b.Print(string(a.output))
 	a.output = nil
 }