diff --git a/internal/fetch/load.go b/internal/fetch/load.go
index be5b7ad..638331e 100644
--- a/internal/fetch/load.go
+++ b/internal/fetch/load.go
@@ -66,8 +66,19 @@
 	ctx, span := trace.StartSpan(ctx, "fetch.loadPackage")
 	defer span.End()
 	var pkgs []*goPackage
+	// Make a map with all the zip file contents.
+	files := make(map[string][]byte)
+	for _, f := range zipGoFiles {
+		_, name := path.Split(f.Name)
+		b, err := readZipFile(f, MaxFileSize)
+		if err != nil {
+			return nil, err
+		}
+		files[name] = b
+	}
+
 	for _, env := range goEnvs {
-		pkg, err := loadPackageWithBuildContext(ctx, env.GOOS, env.GOARCH, zipGoFiles, innerPath, sourceInfo, modInfo)
+		pkg, err := loadPackageWithBuildContext(ctx, env.GOOS, env.GOARCH, files, innerPath, sourceInfo, modInfo)
 		if err != nil && !errors.Is(err, godoc.ErrTooLarge) && !errors.Is(err, derrors.NotFound) {
 			return nil, err
 		}
@@ -126,12 +137,12 @@
 // or all .go files have been excluded by constraints.
 // A *BadPackageError error is returned if the directory
 // contains .go files but do not make up a valid package.
-func loadPackageWithBuildContext(ctx context.Context, goos, goarch string, zipGoFiles []*zip.File, innerPath string, sourceInfo *source.Info, modInfo *godoc.ModuleInfo) (_ *goPackage, err error) {
+func loadPackageWithBuildContext(ctx context.Context, goos, goarch string, files map[string][]byte, innerPath string, sourceInfo *source.Info, modInfo *godoc.ModuleInfo) (_ *goPackage, err error) {
 	modulePath := modInfo.ModulePath
-	defer derrors.Wrap(&err, "loadPackageWithBuildContext(%q, %q, zipGoFiles, %q, %q, %+v)",
+	defer derrors.Wrap(&err, "loadPackageWithBuildContext(%q, %q, files, %q, %q, %+v)",
 		goos, goarch, innerPath, modulePath, sourceInfo)
 
-	packageName, goFiles, fset, err := loadFilesWithBuildContext(innerPath, goos, goarch, zipGoFiles)
+	packageName, goFiles, fset, err := loadFilesWithBuildContext(innerPath, goos, goarch, files)
 	if err != nil {
 		return nil, err
 	}
@@ -180,9 +191,9 @@
 // and goarch in the zip. It returns the package name as it occurs in the
 // source, a map of the ASTs of all the Go files, and the token.FileSet used for
 // parsing.
-func loadFilesWithBuildContext(innerPath, goos, goarch string, zipGoFiles []*zip.File) (pkgName string, fileMap map[string]*ast.File, _ *token.FileSet, _ error) {
+func loadFilesWithBuildContext(innerPath, goos, goarch string, allFiles map[string][]byte) (pkgName string, fileMap map[string]*ast.File, _ *token.FileSet, _ error) {
 	// Apply build constraints to get a map from matching file names to their contents.
-	files, err := matchingFiles(goos, goarch, zipGoFiles)
+	files, err := matchingFiles(goos, goarch, allFiles)
 	if err != nil {
 		return "", nil, nil, err
 	}
@@ -232,18 +243,8 @@
 
 // matchingFiles returns a map from file names to their contents, read from zipGoFiles.
 // It includes only those files that match the build context determined by goos and goarch.
-func matchingFiles(goos, goarch string, zipGoFiles []*zip.File) (files map[string][]byte, err error) {
+func matchingFiles(goos, goarch string, allFiles map[string][]byte) (matchedFiles map[string][]byte, err error) {
 	defer derrors.Wrap(&err, "matchingFiles(%q, %q, zipGoFiles)", goos, goarch)
-	// Populate the map with all the zip files.
-	files = make(map[string][]byte)
-	for _, f := range zipGoFiles {
-		_, name := path.Split(f.Name)
-		b, err := readZipFile(f, MaxFileSize)
-		if err != nil {
-			return nil, err
-		}
-		files[name] = b
-	}
 
 	// bctx is used to make decisions about which of the .go files are included
 	// by build constraints.
@@ -256,7 +257,7 @@
 
 		JoinPath: path.Join,
 		OpenFile: func(name string) (io.ReadCloser, error) {
-			return ioutil.NopCloser(bytes.NewReader(files[name])), nil
+			return ioutil.NopCloser(bytes.NewReader(allFiles[name])), nil
 		},
 
 		// If left nil, the default implementations of these read from disk,
@@ -270,17 +271,21 @@
 		ReadDir:       func(string) ([]os.FileInfo, error) { panic("internal error: unexpected call to ReadDir") },
 	}
 
-	for name := range files {
+	// Copy the input map so we don't modify it.
+	matchedFiles = map[string][]byte{}
+	for n, c := range allFiles {
+		matchedFiles[n] = c
+	}
+	for name := range allFiles {
 		match, err := bctx.MatchFile(".", name) // This will access the file we just added to files map above.
 		if err != nil {
 			return nil, &BadPackageError{Err: fmt.Errorf(`bctx.MatchFile(".", %q): %w`, name, err)}
 		}
 		if !match {
-			// Excluded by build context.
-			delete(files, name)
+			delete(matchedFiles, name)
 		}
 	}
-	return files, nil
+	return matchedFiles, nil
 }
 
 // readZipFile decompresses zip file f and returns its uncompressed contents.
diff --git a/internal/fetch/load_test.go b/internal/fetch/load_test.go
index 166a369..21bfbd3 100644
--- a/internal/fetch/load_test.go
+++ b/internal/fetch/load_test.go
@@ -5,8 +5,6 @@
 package fetch
 
 import (
-	"archive/zip"
-	"bytes"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -28,15 +26,15 @@
 		type Value int`
 
 	plainContents := map[string]string{
-		"README.md":      "THIS IS A README",
-		"LICENSE.md":     testhelper.MITLicense,
-		"plain/plain.go": plainGoBody,
+		"README.md":  "THIS IS A README",
+		"LICENSE.md": testhelper.MITLicense,
+		"plain.go":   plainGoBody,
 	}
 
 	jsContents := map[string]string{
 		"README.md":  "THIS IS A README",
 		"LICENSE.md": testhelper.MITLicense,
-		"js/js.go":   jsGoBody,
+		"js.go":      jsGoBody,
 	}
 	for _, test := range []struct {
 		name         string
@@ -80,15 +78,11 @@
 		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
-			data, err := testhelper.ZipContents(test.contents)
-			if err != nil {
-				t.Fatal(err)
+			files := map[string][]byte{}
+			for n, c := range test.contents {
+				files[n] = []byte(c)
 			}
-			r, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
-			if err != nil {
-				t.Fatal(err)
-			}
-			got, err := matchingFiles(test.goos, test.goarch, r.File)
+			got, err := matchingFiles(test.goos, test.goarch, files)
 			if err != nil {
 				t.Fatal(err)
 			}
diff --git a/internal/fetch/package.go b/internal/fetch/package.go
index 8acee3b..314bef3 100644
--- a/internal/fetch/package.go
+++ b/internal/fetch/package.go
@@ -108,7 +108,7 @@
 	// only after we're sure this phase passed without errors.
 	for _, f := range r.File {
 		if f.Mode().IsDir() {
-			// While "go mod download" will never put a directory in a zip, any can serve their
+			// While "go mod download" will never put a directory in a zip, anyone can serve their
 			// own zips. Example: go.felesatra.moe/binpack@v0.1.0.
 			// Directory entries are harmless, so we just ignore them.
 			continue