diff --git a/custom_completions_test.go b/custom_completions_test.go
index 7cd59e7bb..65b76e740 100644
--- a/custom_completions_test.go
+++ b/custom_completions_test.go
@@ -720,17 +720,17 @@ func TestRequiredFlagNameCompletionInGo(t *testing.T) {
 	rootCmd.AddCommand(childCmd)
 
 	rootCmd.Flags().IntP("requiredFlag", "r", -1, "required flag")
-	rootCmd.MarkFlagRequired("requiredFlag")
+	er(rootCmd.MarkFlagRequired("requiredFlag"))
 	requiredFlag := rootCmd.Flags().Lookup("requiredFlag")
 
 	rootCmd.PersistentFlags().IntP("requiredPersistent", "p", -1, "required persistent")
-	rootCmd.MarkPersistentFlagRequired("requiredPersistent")
+	er(rootCmd.MarkPersistentFlagRequired("requiredPersistent"))
 	requiredPersistent := rootCmd.PersistentFlags().Lookup("requiredPersistent")
 
 	rootCmd.Flags().StringP("release", "R", "", "Release name")
 
 	childCmd.Flags().BoolP("subRequired", "s", false, "sub required flag")
-	childCmd.MarkFlagRequired("subRequired")
+	er(childCmd.MarkFlagRequired("subRequired"))
 	childCmd.Flags().BoolP("subNotRequired", "n", false, "sub not required flag")
 
 	// Test that a required flag is suggested even without the - prefix
@@ -904,19 +904,19 @@ func TestFlagFileExtFilterCompletionInGo(t *testing.T) {
 
 	// No extensions.  Should be ignored.
 	rootCmd.Flags().StringP("file", "f", "", "file flag")
-	rootCmd.MarkFlagFilename("file")
+	er(rootCmd.MarkFlagFilename("file"))
 
 	// Single extension
 	rootCmd.Flags().StringP("log", "l", "", "log flag")
-	rootCmd.MarkFlagFilename("log", "log")
+	er(rootCmd.MarkFlagFilename("log", "log"))
 
 	// Multiple extensions
 	rootCmd.Flags().StringP("yaml", "y", "", "yaml flag")
-	rootCmd.MarkFlagFilename("yaml", "yaml", "yml")
+	er(rootCmd.MarkFlagFilename("yaml", "yaml", "yml"))
 
 	// Directly using annotation
 	rootCmd.Flags().StringP("text", "t", "", "text flag")
-	rootCmd.Flags().SetAnnotation("text", BashCompFilenameExt, []string{"txt"})
+	er(rootCmd.Flags().SetAnnotation("text", BashCompFilenameExt, []string{"txt"}))
 
 	// Test that the completion logic returns the proper info for the completion
 	// script to handle the file filtering
@@ -1026,15 +1026,15 @@ func TestFlagDirFilterCompletionInGo(t *testing.T) {
 
 	// Filter directories
 	rootCmd.Flags().StringP("dir", "d", "", "dir flag")
-	rootCmd.MarkFlagDirname("dir")
+	er(rootCmd.MarkFlagDirname("dir"))
 
 	// Filter directories within a directory
 	rootCmd.Flags().StringP("subdir", "s", "", "subdir")
-	rootCmd.Flags().SetAnnotation("subdir", BashCompSubdirsInDir, []string{"themes"})
+	er(rootCmd.Flags().SetAnnotation("subdir", BashCompSubdirsInDir, []string{"themes"}))
 
 	// Multiple directory specification get ignored
 	rootCmd.Flags().StringP("manydir", "m", "", "manydir")
-	rootCmd.Flags().SetAnnotation("manydir", BashCompSubdirsInDir, []string{"themes", "colors"})
+	er(rootCmd.Flags().SetAnnotation("manydir", BashCompSubdirsInDir, []string{"themes", "colors"}))
 
 	// Test that the completion logic returns the proper info for the completion
 	// script to handle the directory filtering
@@ -1417,7 +1417,7 @@ func TestCompleteNoDesCmdInZshScript(t *testing.T) {
 	rootCmd.AddCommand(child)
 
 	buf := new(bytes.Buffer)
-	rootCmd.GenZshCompletionNoDesc(buf)
+	er(rootCmd.GenZshCompletionNoDesc(buf))
 	output := buf.String()
 
 	check(t, output, ShellCompNoDescRequestCmd)
@@ -1433,7 +1433,7 @@ func TestCompleteCmdInZshScript(t *testing.T) {
 	rootCmd.AddCommand(child)
 
 	buf := new(bytes.Buffer)
-	rootCmd.GenZshCompletion(buf)
+	er(rootCmd.GenZshCompletion(buf))
 	output := buf.String()
 
 	check(t, output, ShellCompRequestCmd)
diff --git a/fish_completions_test.go b/fish_completions_test.go
index 532b6055a..c9b4205f0 100644
--- a/fish_completions_test.go
+++ b/fish_completions_test.go
@@ -15,7 +15,7 @@ func TestCompleteNoDesCmdInFishScript(t *testing.T) {
 	rootCmd.AddCommand(child)
 
 	buf := new(bytes.Buffer)
-	rootCmd.GenFishCompletion(buf, false)
+	er(rootCmd.GenFishCompletion(buf, false))
 	output := buf.String()
 
 	check(t, output, ShellCompNoDescRequestCmd)
@@ -31,7 +31,7 @@ func TestCompleteCmdInFishScript(t *testing.T) {
 	rootCmd.AddCommand(child)
 
 	buf := new(bytes.Buffer)
-	rootCmd.GenFishCompletion(buf, true)
+	er(rootCmd.GenFishCompletion(buf, true))
 	output := buf.String()
 
 	check(t, output, ShellCompRequestCmd)
@@ -41,7 +41,7 @@ func TestCompleteCmdInFishScript(t *testing.T) {
 func TestProgWithDash(t *testing.T) {
 	rootCmd := &Command{Use: "root-dash", Args: NoArgs, Run: emptyRun}
 	buf := new(bytes.Buffer)
-	rootCmd.GenFishCompletion(buf, false)
+	er(rootCmd.GenFishCompletion(buf, false))
 	output := buf.String()
 
 	// Functions name should have replace the '-'
diff --git a/zsh_completions.go b/zsh_completions.go
index c25ce680c..914d6e424 100644
--- a/zsh_completions.go
+++ b/zsh_completions.go
@@ -70,12 +70,12 @@ func (c *Command) genZshCompletion(w io.Writer, includeDesc bool) error {
 	return err
 }
 
-func genZshComp(buf *bytes.Buffer, name string, includeDesc bool) {
+func genZshComp(buf io.StringWriter, name string, includeDesc bool) {
 	compCmd := ShellCompRequestCmd
 	if !includeDesc {
 		compCmd = ShellCompNoDescRequestCmd
 	}
-	buf.WriteString(fmt.Sprintf(`#compdef _%[1]s %[1]s
+	WrStringAndCheck(buf, fmt.Sprintf(`#compdef _%[1]s %[1]s
 
 # zsh completion for %-36[1]s -*- shell-script -*-