diff --git a/__tests__/fixtures/mockPlugins/valid-plugin/functions.js b/__tests__/fixtures/mockPlugins/valid-plugin/functions.js
new file mode 100644
index 0000000000..59ae812eac
--- /dev/null
+++ b/__tests__/fixtures/mockPlugins/valid-plugin/functions.js
@@ -0,0 +1 @@
+// intentionally empty, file must just exist for testing purposes.
diff --git a/__tests__/fixtures/mockPlugins/valid-plugin/govuk-prototype-kit.config.json b/__tests__/fixtures/mockPlugins/valid-plugin/govuk-prototype-kit.config.json
index afec4488ff..770e2f47dd 100644
--- a/__tests__/fixtures/mockPlugins/valid-plugin/govuk-prototype-kit.config.json
+++ b/__tests__/fixtures/mockPlugins/valid-plugin/govuk-prototype-kit.config.json
@@ -23,6 +23,7 @@
 		}
 	  ],
 	"nunjucksFilters": "/filters.js",
+	"nunjucksFunctions": "/functions.js",
 	"templates": [
 	  {
 		"name": "Step by step navigation page",
@@ -42,4 +43,4 @@
 		  "maxVersion": "5.5.0"
 	  }
 	]
-  }
\ No newline at end of file
+  }
diff --git a/lib/plugins/plugin-validator.js b/lib/plugins/plugin-validator.js
index abb0a337f5..9fb928cbc0 100644
--- a/lib/plugins/plugin-validator.js
+++ b/lib/plugins/plugin-validator.js
@@ -12,6 +12,7 @@ const knownKeys = [
   'nunjucksPaths',
   'nunjucksMacros',
   'nunjucksFilters',
+  'nunjucksFunctions',
   'sass',
   'scripts',
   'stylesheets',