Fixing issue with isValid function found by testcase.

packages/string-templates/src/index.js

@@ -177,7 +177,9 @@ module.exports.isValid = (string, opts) => {
   const context = {}
   try {
     const instance = opts.noHelpers ? hbsInstanceNoHelpers : hbsInstance
-    instance.compile(processors.preprocess(string, false))(context)
+    instance.compile(processors.preprocess(string, { noFinalise: true }))(
+      context
+    )
     return true
   } catch (err) {
     const msg = err && err.message ? err.message : err

packages/string-templates/src/processors/index.js

@@ -23,9 +23,13 @@ function process(output, processors, opts) {
 
 module.exports.preprocess = (string, opts) => {
   let processors = preprocessor.processors
+  if (opts.noFinalise) {
+    processors = processors.filter(
+      processor => processor.name !== preprocessor.PreprocessorNames.FINALISE
+    )
+  }
   return process(string, processors, opts)
 }
-
 module.exports.postprocess = string => {
   let processors = postprocessor.processors
   return process(string, processors)