diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index d0beb6e747..7ae2346b78 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -99,6 +99,7 @@ func Init() {
 	case "on", "":
 		mustUseModules = true
 	case "off":
+		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
 		mustUseModules = false
 		return
 	}