diff --git a/Sources/ArgumentParser/Completions/CompletionsGenerator.swift b/Sources/ArgumentParser/Completions/CompletionsGenerator.swift
index f10749074..213717b5e 100644
--- a/Sources/ArgumentParser/Completions/CompletionsGenerator.swift
+++ b/Sources/ArgumentParser/Completions/CompletionsGenerator.swift
@@ -149,12 +149,13 @@ extension ArgumentDefinition {
   /// Returns a string with the arguments for the callback to generate custom completions for
   /// this argument.
   func customCompletionCall(_ commands: [ParsableCommand.Type]) -> String {
-    let subcommandNames = commands.dropFirst().map { $0._commandName }.joined(
-      separator: " ")
+    let subcommandNames =
+      commands.dropFirst().map { "\($0._commandName) " }.joined()
     let argumentName =
       names.preferredName?.synopsisString
-      ?? self.help.keys.first?.fullPathString ?? "---"
-    return "---completion \(subcommandNames) -- \(argumentName)"
+      ?? self.help.keys.first?.fullPathString
+      ?? "---"
+    return "---completion \(subcommandNames)-- \(argumentName)"
   }
 }
 
diff --git a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Bash().bash b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Bash().bash
index 8da5e1893..6fc57bb40 100644
--- a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Bash().bash
+++ b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Bash().bash
@@ -182,11 +182,11 @@ _base_test() {
     # Offer positional completions
     case "${positional_number}" in
     1)
-        __base_test_add_completions -W "$(__base_test_custom_complete ---completion  -- argument)"
+        __base_test_add_completions -W "$(__base_test_custom_complete ---completion -- argument)"
         return
         ;;
     2)
-        __base_test_add_completions -W "$(__base_test_custom_complete ---completion  -- nested.nestedArgument)"
+        __base_test_add_completions -W "$(__base_test_custom_complete ---completion -- nested.nestedArgument)"
         return
         ;;
     esac
diff --git a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Fish().fish b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Fish().fish
index 03a03de7b..ac32cce50 100644
--- a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Fish().fish
+++ b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Fish().fish
@@ -80,8 +80,8 @@ complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test"
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test"' -l kind-counter
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test"' -l rep1 -rfka ''
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test"' -s r -l rep2 -rfka ''
-complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 1' -fka '(__base-test_custom_completion ---completion  -- argument)'
-complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 2' -fka '(__base-test_custom_completion ---completion  -- nested.nestedArgument)'
+complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 1' -fka '(__base-test_custom_completion ---completion -- argument)'
+complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 2' -fka '(__base-test_custom_completion ---completion -- nested.nestedArgument)'
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test"' -s h -l help -d 'Show help information.'
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 3' -fa 'sub-command' -d ''
 complete -c 'base-test' -n '__base-test_should_offer_completions_for "base-test" 3' -fa 'escaped-command' -d ''
diff --git a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Zsh().zsh b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Zsh().zsh
index 0b0762c78..6a046fb81 100644
--- a/Tests/ArgumentParserUnitTests/Snapshots/testBase_Zsh().zsh
+++ b/Tests/ArgumentParserUnitTests/Snapshots/testBase_Zsh().zsh
@@ -47,8 +47,8 @@ _base-test() {
         '*--kind-counter'
         '*--rep1:rep1:'
         '*'{-r,--rep2}':rep2:'
-        ':argument:{__base-test_custom_complete "${command_name}" ---completion  -- argument "${command_line[@]}"}'
-        ':nested-argument:{__base-test_custom_complete "${command_name}" ---completion  -- nested.nestedArgument "${command_line[@]}"}'
+        ':argument:{__base-test_custom_complete "${command_name}" ---completion -- argument "${command_line[@]}"}'
+        ':nested-argument:{__base-test_custom_complete "${command_name}" ---completion -- nested.nestedArgument "${command_line[@]}"}'
         '(-h --help)'{-h,--help}'[Show help information.]'
         '(-): :->command'
         '(-)*:: :->arg'