diff --git a/fs/asyncreader/asyncreader_test.go b/fs/asyncreader/asyncreader_test.go
index 2a5e06099..4aab06604 100644
--- a/fs/asyncreader/asyncreader_test.go
+++ b/fs/asyncreader/asyncreader_test.go
@@ -146,7 +146,7 @@ func TestAsyncReaderSizes(t *testing.T) {
 	for i := 0; i < len(texts)-1; i++ {
 		texts[i] = str + "\n"
 		all += texts[i]
-		str += string(i%26 + 'a')
+		str += string(rune(i)%26 + 'a')
 	}
 	texts[len(texts)-1] = all
 
@@ -185,7 +185,7 @@ func TestAsyncReaderWriteTo(t *testing.T) {
 	for i := 0; i < len(texts)-1; i++ {
 		texts[i] = str + "\n"
 		all += texts[i]
-		str += string(i%26 + 'a')
+		str += string(rune(i)%26 + 'a')
 	}
 	texts[len(texts)-1] = all
 
diff --git a/fs/walk/walk_test.go b/fs/walk/walk_test.go
index 373ffe753..e50e9478a 100644
--- a/fs/walk/walk_test.go
+++ b/fs/walk/walk_test.go
@@ -358,7 +358,7 @@ func walkErrors(t *testing.T, expectedErr error) *listDirs {
 	em := errorMap{}
 	de := make(fs.DirEntries, 10)
 	for i := range de {
-		path := string('0' + i)
+		path := string('0' + rune(i))
 		de[i] = mockdir.New(path)
 		lr[path] = listResult{entries: nil, err: fs.ErrorDirNotFound}
 		em[path] = fs.ErrorDirNotFound