SetPlatformTextureSettings: Selected texture format 'Unsupported' for platform 'iPhone'

Why do I get this error:

Selected texture format 'Unsupported' for platform 'iPhone' is not valid with the current texture type 'Sprite'.

When I use this code to auto set my texture import settings?

 static void OptimizeFor_iPhone(string path) {
     var texture_importer = AssetImporter.GetAtPath(path) as TextureImporter;
     var platform_import_settings = texture_importer.GetPlatformTextureSettings("iPhone");

     platform_import_settings.maxTextureSize = Mathf.Min(texture_importer.maxTextureSize, platform_import_settings.maxTextureSize);
     platform_import_settings.format = TextureImporterFormat.ASTC_RGB_6x6;
     platform_import_settings.overridden = true; 

     // Callstack from error says it comes from this line:
     texture_importer.SetPlatformTextureSettings(platform_import_settings);

     AssetDatabase.WriteImportSettingsIfDirty(path);
 }

(That code is run on the results of AssetDatabase.FindAssets("t:texture ", folders) to optimize texture settings on a folder.)

It looks like the code successfully changes the import settings (a section with "buildTarget: iPhone" and "textureFormat: 56" is added to the png's .meta file), so I don't understand what the error is trying to tell me.

I tried Windows (no addons) and macOS (Unity's iPhone and tvOS addons installed) and got the same error. After running the code, the Texture Import Inspector looks as expected.