diff --git a/bash_completions.go b/bash_completions.go
index 925e6e787..733f4d121 100644
--- a/bash_completions.go
+++ b/bash_completions.go
@@ -512,7 +512,9 @@ func writeLocalNonPersistentFlag(buf io.StringWriter, flag *pflag.Flag) {
 
 // Setup annotations for go completions for registered flags
 func prepareCustomAnnotationsForFlags(cmd *Command) {
-	for flag := range cmd.Root().flagCompletionFunctions {
+	flagCompletionMutex.RLock()
+	defer flagCompletionMutex.RUnlock()
+	for flag := range flagCompletionFunctions {
 		// Make sure the completion script calls the __*_go_custom_completion function for
 		// every registered flag.  We need to do this here (and not when the flag was registered
 		// for completion) so that we can know the root command name for the prefix
diff --git a/command.go b/command.go
index 9f33a461f..2cc18891d 100644
--- a/command.go
+++ b/command.go
@@ -142,9 +142,6 @@ type Command struct {
 	// that we can use on every pflag set and children commands
 	globNormFunc func(f *flag.FlagSet, name string) flag.NormalizedName
 
-	//flagCompletionFunctions is map of flag completion functions.
-	flagCompletionFunctions map[*flag.Flag]func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective)
-
 	// usageFunc is usage func defined by user.
 	usageFunc func(*Command) error
 	// usageTemplate is usage template defined by user.
diff --git a/completions.go b/completions.go
index 4687674aa..b849b9c84 100644
--- a/completions.go
+++ b/completions.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"sync"
 
 	"github.com/spf13/pflag"
 )
@@ -17,6 +18,12 @@ const (
 	ShellCompNoDescRequestCmd = "__completeNoDesc"
 )
 
+// Global map of flag completion functions. Make sure to use flagCompletionMutex before you try to read and write from it.
+var flagCompletionFunctions = map[*pflag.Flag]func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective){}
+
+// lock for reading and writing from flagCompletionFunctions
+var flagCompletionMutex = &sync.RWMutex{}
+
 // ShellCompDirective is a bit map representing the different behaviors the shell
 // can be instructed to have once completions have been provided.
 type ShellCompDirective int
@@ -100,15 +107,13 @@ func (c *Command) RegisterFlagCompletionFunc(flagName string, f func(cmd *Comman
 	if flag == nil {
 		return fmt.Errorf("RegisterFlagCompletionFunc: flag '%s' does not exist", flagName)
 	}
+	flagCompletionMutex.Lock()
+	defer flagCompletionMutex.Unlock()
 
-	root := c.Root()
-	if _, exists := root.flagCompletionFunctions[flag]; exists {
+	if _, exists := flagCompletionFunctions[flag]; exists {
 		return fmt.Errorf("RegisterFlagCompletionFunc: flag '%s' already registered", flagName)
 	}
-	if root.flagCompletionFunctions == nil {
-		root.flagCompletionFunctions = map[*pflag.Flag]func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective){}
-	}
-	root.flagCompletionFunctions[flag] = f
+	flagCompletionFunctions[flag] = f
 	return nil
 }
 
@@ -402,7 +407,9 @@ func (c *Command) getCompletions(args []string) (*Command, []string, ShellCompDi
 	// Find the completion function for the flag or command
 	var completionFn func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective)
 	if flag != nil && flagCompletion {
-		completionFn = c.Root().flagCompletionFunctions[flag]
+		flagCompletionMutex.RLock()
+		completionFn = flagCompletionFunctions[flag]
+		flagCompletionMutex.RUnlock()
 	} else {
 		completionFn = finalCmd.ValidArgsFunction
 	}
diff --git a/completions_test.go b/completions_test.go
index aea06a241..70c455af1 100644
--- a/completions_test.go
+++ b/completions_test.go
@@ -1971,6 +1971,41 @@ func TestFlagCompletionWithNotInterspersedArgs(t *testing.T) {
 	}
 }
 
+func TestFlagCompletionWorksRootCommandAddedAfterFlags(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	childCmd := &Command{
+		Use: "child",
+		Run: emptyRun,
+		ValidArgsFunction: func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+			return []string{"--validarg", "test"}, ShellCompDirectiveDefault
+		},
+	}
+	childCmd.Flags().Bool("bool", false, "test bool flag")
+	childCmd.Flags().String("string", "", "test string flag")
+	_ = childCmd.RegisterFlagCompletionFunc("string", func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return []string{"myval"}, ShellCompDirectiveDefault
+	})
+
+	// Important: This is a test for https://github.com/spf13/cobra/issues/1437
+	// Only add the subcommand after RegisterFlagCompletionFunc was called, do not change this order!
+	rootCmd.AddCommand(childCmd)
+
+	// Test that flag completion works for the subcmd
+	output, err := executeCommand(rootCmd, ShellCompRequestCmd, "child", "--string", "")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	expected := strings.Join([]string{
+		"myval",
+		":0",
+		"Completion ended with directive: ShellCompDirectiveDefault", ""}, "\n")
+
+	if output != expected {
+		t.Errorf("expected: %q, got: %q", expected, output)
+	}
+}
+
 func TestFlagCompletionInGoWithDesc(t *testing.T) {
 	rootCmd := &Command{
 		Use: "root",
diff --git a/fish_completions.go b/fish_completions.go
index b0db323ac..bb57fd568 100644
--- a/fish_completions.go
+++ b/fish_completions.go
@@ -152,11 +152,11 @@ function __%[1]s_prepare_completions
             # We don't need descriptions anyway since there is only a single
             # real completion which the shell will expand immediately.
             set -l split (string split --max 1 \t $__%[1]s_comp_results[1])
-        
+
             # Fish won't add a space if the completion ends with any
             # of the following characters: @=/:.,
             set -l lastChar (string sub -s -1 -- $split)
-            if not string match -r -q "[@=/:.,]" -- "$lastChar"                
+            if not string match -r -q "[@=/:.,]" -- "$lastChar"
                 # In other cases, to support the "nospace" directive we trick the shell
                 # by outputting an extra, longer completion.
                 __%[1]s_debug "Adding second completion to perform nospace directive"