diff --git a/application/helpers/SurveyRuntimeHelper.php b/application/helpers/SurveyRuntimeHelper.php
index 1225f8f24e6..e9d23d8b8cb 100644
--- a/application/helpers/SurveyRuntimeHelper.php
+++ b/application/helpers/SurveyRuntimeHelper.php
@@ -1382,7 +1382,7 @@ private function moveSubmitIfNeeded()
 
     /**
      * Check in a string if it uses expressions to replace them
-     * @param string $sString the string to evaluate
+     * @param string|null $sString the string to evaluate
      * @param integer $numRecursionLevels - the number of times to recursively subtitute values in this string
      * @param boolean $static - return static string
      * @return string
@@ -1392,7 +1392,7 @@ private function processString($sString, $iRecursionLevel = 1, $static = false)
     {
         $sProcessedString = $sString;
 
-        if ((strpos($sProcessedString, "{") !== false)) {
+        if ((strpos((string) $sProcessedString, "{") !== false)) {
             // process string anyway so that it can be pretty-printed
             $aStandardsReplacementFields = getStandardsReplacementFields($this->aSurveyInfo);
             $sProcessedString = LimeExpressionManager::ProcessStepString($sString, $aStandardsReplacementFields, $iRecursionLevel, $static);