diff --git a/autoload/codefmt/formatterhelpers.vim b/autoload/codefmt/formatterhelpers.vim
index 43f06c9..a0a335d 100644
--- a/autoload/codefmt/formatterhelpers.vim
+++ b/autoload/codefmt/formatterhelpers.vim
@@ -13,6 +13,9 @@
 " limitations under the License.
 
 
+let s:plugin = maktaba#plugin#Get('codefmt')
+
+
 ""
 " @public
 " Format lines in the current buffer via a formatter invoked by {cmd}, which
@@ -31,6 +34,7 @@ function! codefmt#formatterhelpers#Format(cmd) abort
   call maktaba#buffer#Overwrite(1, line('$'), l:formatted)
 endfunction
 
+
 ""
 " @public
 " Attempt to format a range of lines from {startline} to {endline} in the
@@ -62,3 +66,44 @@ function! codefmt#formatterhelpers#AttemptFakeRangeFormatting(
 
   call maktaba#buffer#Overwrite(1, line('$'), l:full_formatted)
 endfunction
+
+
+""
+" @public
+" Resolve a flag (function, string or array) to a normalized array, with special
+" handling to convert a spaceless string to a single-element array. This is the
+" common case for executables, and more importantly, is backward-compatible for
+" existing user settings.
+"
+" @throws WrongType if the flag doesn't resolve to a string or array
+function! codefmt#formatterhelpers#ResolveFlagToArray(flag_name) abort
+  let l:FlagFn = s:plugin.Flag(a:flag_name)
+  if maktaba#value#IsFuncref(l:FlagFn)
+    let l:value = maktaba#function#Call(l:FlagFn)
+  else
+    let l:value = l:FlagFn
+  endif
+
+  " After (conditionally) calling the function, the resulting value should be
+  " either a list that we can use directly, or a string that we can treat as
+  " a single-element list, mainly for backward compatibility.
+  if maktaba#value#IsString(l:value)
+    if l:value =~ '\s'
+      " Uh oh, there are spaces in the string. Rather than guessing user intent
+      " with shell quoting and word splitting, handle this (hopefully unusual)
+      " case by telling them to update their configuration.
+      throw maktaba#error#WrongType(
+            \ '%s flag is a string with spaces, please make it a list. ' .
+            \ 'Resolved value was: %s',
+            \ a:flag_name, l:value)
+    endif
+    " Convert spaceless string to single-element list.
+    let l:value = [l:value]
+  elseif !maktaba#value#IsList(l:value)
+    throw maktaba#error#WrongType(
+          \ '%s flag should be a list after calling. Found %s',
+          \ a:flag_name, maktaba#value#TypeName(l:value))
+  endif
+
+  return l:value
+endfunction
diff --git a/autoload/codefmt/prettier.vim b/autoload/codefmt/prettier.vim
index 70399f0..dd25299 100644
--- a/autoload/codefmt/prettier.vim
+++ b/autoload/codefmt/prettier.vim
@@ -16,7 +16,7 @@
 let s:plugin = maktaba#plugin#Get('codefmt')
 
 " See https://prettier.io for a list of supported file types.
-let s:supported_filetypes = ['javascript', 'markdown', 'html', 'css', 'yaml', 
+let s:supported_filetypes = ['javascript', 'markdown', 'html', 'css', 'yaml',
       \ 'jsx', 'less', 'scss', 'mdx', 'vue']
 
 
@@ -30,7 +30,9 @@ function! codefmt#prettier#GetFormatter() abort
           \ 'and configure the prettier_executable flag'}
 
   function l:formatter.IsAvailable() abort
-    return executable(s:plugin.Flag('prettier_executable'))
+    let l:cmd = codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'prettier_executable')
+    return !empty(l:cmd) && executable(l:cmd[0])
   endfunction
 
   function l:formatter.AppliesToBuffer() abort
@@ -42,17 +44,8 @@ function! codefmt#prettier#GetFormatter() abort
   " @flag(prettier_executable), only targeting the range between {startline} and
   " {endline}.
   function l:formatter.FormatRange(startline, endline) abort
-    let l:Prettier_options = s:plugin.Flag('prettier_options')
-    if type(l:Prettier_options) is# type([])
-      let l:prettier_options = l:Prettier_options
-    elseif maktaba#value#IsCallable(l:Prettier_options)
-      let l:prettier_options = maktaba#function#Call(l:Prettier_options)
-    else
-      throw maktaba#error#WrongType(
-          \ 'prettier_options flag must be list or callable. Found %s',
-          \ string(l:Prettier_options))
-    endif
-    let l:cmd = [s:plugin.Flag('prettier_executable'), '--stdin', '--no-color']
+    let l:cmd = codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'prettier_executable') + ['--stdin', '--no-color']
 
     " prettier is able to automatically choose the best parser if the filepath
     " is provided. Otherwise, fall back to the previous default: babylon.
@@ -74,7 +67,9 @@ function! codefmt#prettier#GetFormatter() abort
     let l:lines_end = join(l:lines[0 : a:endline - 1], "\n")
     call extend(l:cmd, ['--range-end', string(strchars(l:lines_end))])
 
-    call extend(l:cmd, l:prettier_options)
+    call extend(l:cmd, codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'prettier_options'))
+
     try
       let l:result = maktaba#syscall#Create(l:cmd).WithStdin(l:input).Call()
       let l:formatted = split(l:result.stdout, "\n")
diff --git a/autoload/codefmt/zprint.vim b/autoload/codefmt/zprint.vim
index bc0a287..3e60014 100644
--- a/autoload/codefmt/zprint.vim
+++ b/autoload/codefmt/zprint.vim
@@ -38,7 +38,9 @@ function! codefmt#zprint#GetFormatter() abort
         \ 'and configure the zprint_executable flag'}
 
   function l:formatter.IsAvailable() abort
-    return executable(s:plugin.Flag('zprint_executable'))
+    let l:cmd = codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'zprint_executable')
+    return !empty(l:cmd) && executable(l:cmd[0])
   endfunction
 
   function l:formatter.AppliesToBuffer() abort
@@ -50,25 +52,15 @@ function! codefmt#zprint#GetFormatter() abort
   " @flag(zprint_executable), only targeting the range between {startline} and
   " {endline}.
   function l:formatter.FormatRange(startline, endline) abort
-    " Must be upper-cased to call as a function
-    let l:ZprintOptions = s:plugin.Flag('zprint_options')
-    if type(l:ZprintOptions) is# type([])
-      " Assign upper-case to lower-case
-      let l:zprint_options = l:ZprintOptions
-    elseif maktaba#value#IsCallable(l:ZprintOptions)
-      " Call upper-case to assign lower-case
-      let l:zprint_options = maktaba#function#Call(l:ZprintOptions)
-    else
-      throw maktaba#error#WrongType(
-            \ 'zprint_options flag must be list or callable. Found %s',
-            \ string(l:ZprintOptions))
-    endif
-    let l:cmd_args = [s:plugin.Flag('zprint_executable')]
-    call extend(l:cmd_args, l:zprint_options)
+    let l:exe = codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'zprint_executable')
+    let l:opts = codefmt#formatterhelpers#ResolveFlagToArray(
+          \ 'zprint_options')
 
     " Prepare the syscall, changing to the containing directory in case the user
     " has configured {:search-config? true} in ~/.zprintrc
-    let l:cmd = maktaba#syscall#Create(l:cmd_args).WithCwd(expand('%:p:h'))
+    let l:cmd = maktaba#syscall#Create(l:exe + l:opts).WithCwd(expand('%:p:h'))
+
     " zprint does not support range formatting yet:
     " https://github.com/kkinnear/zprint/issues/122
     " This fake range formatting works well for top-level forms, although it's