diff --git a/src/cmd/go/internal/modload/search.go b/src/cmd/go/internal/modload/search.go
index 2e82b92cc5..2cd657326c 100644
--- a/src/cmd/go/internal/modload/search.go
+++ b/src/cmd/go/internal/modload/search.go
@@ -105,7 +105,14 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 	}
 
 	if cfg.BuildMod == "vendor" {
-		walkPkgs(filepath.Join(ModRoot(), "vendor"), "", false)
+		if HasModRoot() {
+			modPrefix := Target.Path
+			if Target.Path == "std" {
+				modPrefix = ""
+			}
+			walkPkgs(ModRoot(), modPrefix, false)
+			walkPkgs(filepath.Join(ModRoot(), "vendor"), "", false)
+		}
 		return pkgs
 	}
 
diff --git a/src/cmd/go/testdata/script/mod_vendor_build.txt b/src/cmd/go/testdata/script/mod_vendor_build.txt
index 01ee2d202a..da3fd91055 100644
--- a/src/cmd/go/testdata/script/mod_vendor_build.txt
+++ b/src/cmd/go/testdata/script/mod_vendor_build.txt
@@ -25,6 +25,10 @@ env GOPROXY=off
 ! go list ...
 go list -mod=vendor ...
 
+# However, it should still list packages in the main module.
+go list -mod=vendor m/...
+stdout m
+
 -- go.mod --
 module m