diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 0bf10e2560a..f42ec50006a 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -317,6 +317,13 @@ func runBuild(cmd *base.Command, args []string) {
 
 	pkgs := load.PackagesForBuild(args)
 
+	explicitO := len(cfg.BuildO) > 0
+
+	if len(pkgs) == 1 && pkgs[0].Name == "main" && cfg.BuildO == "" {
+		cfg.BuildO = pkgs[0].DefaultExecName()
+		cfg.BuildO += cfg.ExeSuffix
+	}
+
 	// sanity check some often mis-used options
 	switch cfg.BuildContext.Compiler {
 	case "gccgo":
@@ -342,11 +349,16 @@ func runBuild(cmd *base.Command, args []string) {
 	// Special case -o /dev/null by not writing at all.
 	if cfg.BuildO == os.DevNull {
 		cfg.BuildO = ""
-	} else if cfg.BuildO != "" {
+	}
+
+	if cfg.BuildO != "" {
 		// If the -o name exists and is a directory, then
 		// write all main packages to that directory.
 		// Otherwise require only a single package be built.
 		if fi, err := os.Stat(cfg.BuildO); err == nil && fi.IsDir() {
+			if !explicitO {
+				base.Fatalf("go build: build output %q already exists and is a directory", cfg.BuildO)
+			}
 			a := &Action{Mode: "go build"}
 			for _, p := range pkgs {
 				if p.Name != "main" {
@@ -377,14 +389,6 @@ func runBuild(cmd *base.Command, args []string) {
 		a := b.AutoAction(ModeInstall, depMode, p)
 		b.Do(a)
 		return
-	} else if len(pkgs) == 1 && pkgs[0].Name == "main" {
-		p := pkgs[0]
-		p.Target = p.DefaultExecName()
-		p.Target += cfg.ExeSuffix
-		p.Stale = true
-		a := b.AutoAction(ModeInstall, depMode, p)
-		b.Do(a)
-		return
 	}
 
 	a := &Action{Mode: "go build"}