diff --git a/includes/exporters/webform_exporter_excel_xlsx.inc b/includes/exporters/webform_exporter_excel_xlsx.inc
index 072a0b3..cb3a1e0 100644
--- a/includes/exporters/webform_exporter_excel_xlsx.inc
+++ b/includes/exporters/webform_exporter_excel_xlsx.inc
@@ -20,79 +20,81 @@ class webform_exporter_excel_xlsx extends webform_exporter {
     $output = '<row>';
     $utc_timezone = new DateTimeZone('UTC');
     foreach ($data as $key => $value) {
-      // Strip UTF8 characters that are not legal in XML files.
-      // See http://www.w3.org/TR/xml/#charsets
-      // See http://stackoverflow.com/questions/3466035/how-to-skip-invalid-characters-in-xml-file-using-php
-      // Char ::= #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
-      $value = preg_replace('/[^\x{0009}\x{000a}\x{000d}\x{0020}-\x{D7FF}\x{E000}-\x{FFFD}]+/u', '', $value);
-
-      $cell_position = $col . $row;
-      if (strlen($value) === 0) {
-        // Skip empty cells.
-      }
-      elseif (is_numeric($value) && $value[0] !== '+') {
-        $output .= '<c r="' . $cell_position . '"><v>';
-        $output .= $value;
-        $output .= '</v></c>';
-      }
-      elseif (preg_match(self::DATE_REGEX_ANY, $value)) {
-        // An Excel timestamp is the number of days since Jan 1, 1900, with
-        // the decimal portion indicating the time (ddddd.tttttt).
-        // To calculate, take the UNIX timestamp then add the number of days
-        // between 1900 and 1970 (25568).
-        $timestamp = date_timestamp_get(date_create($value, $utc_timezone));
-        $excel_timestamp = ($timestamp / 86400) + 25568;
-
-        // 1900 is treated as a leap year, but it is not. So all dates after
-        // Feb 28, 1900 have one extra day added. That is "59" should be
-        // March 1, 1900, but it's considered Feb 29, 1900, which didn't exist.
-        // So all dates greater than 59 have 1 extra day added.
-        // See http://www.cpearson.com/excel/datetime.htm.
-        if ($excel_timestamp >= 59) {
-          $excel_timestamp = $excel_timestamp + 1.0;
-        }
+      if (is_string($value)) {
+        // Strip UTF8 characters that are not legal in XML files.
+        // See http://www.w3.org/TR/xml/#charsets
+        // See http://stackoverflow.com/questions/3466035/how-to-skip-invalid-characters-in-xml-file-using-php
+        // Char ::= #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
+        $value = preg_replace('/[^\x{0009}\x{000a}\x{000d}\x{0020}-\x{D7FF}\x{E000}-\x{FFFD}]+/u', '', $value);
 
-        // Excel does not support dates prior to 0 (Jan 1, 1900). They have to
-        // be represented as plain-text instead.
-        if ($excel_timestamp <= 0) {
-          $output .= '<c r="' . $cell_position . '" t="inlineStr"><is><t>';
-          $output .= htmlspecialchars($value, ENT_QUOTES);
-          $output .= '</t></is></c>';
+        $cell_position = $col . $row;
+        if (strlen($value) === 0) {
+          // Skip empty cells.
         }
-        // Now after calculating the Excel "timestamp", save it as a decimal
-        // and point to a style formatter to make it appear as a date/time.
-        else {
-          // 1: Dates.
-          // 2: Times.
-          // 3: Date times.
-          // These are tied to style definitions in the styles.xml file
-          // generated by webform_exporter_excel_xlsx::xlsx_parts().
-          if (preg_match(self::DATE_REGEX_DATE, $value)) {
-            $style_format = 1;
+        elseif (is_numeric($value) && $value[0] !== '+') {
+          $output .= '<c r="' . $cell_position . '"><v>';
+          $output .= $value;
+          $output .= '</v></c>';
+        }
+        elseif (preg_match(self::DATE_REGEX_ANY, $value)) {
+          // An Excel timestamp is the number of days since Jan 1, 1900, with
+          // the decimal portion indicating the time (ddddd.tttttt).
+          // To calculate, take the UNIX timestamp then add the number of days
+          // between 1900 and 1970 (25568).
+          $timestamp = date_timestamp_get(date_create($value, $utc_timezone));
+          $excel_timestamp = ($timestamp / 86400) + 25568;
+
+          // 1900 is treated as a leap year, but it is not. So all dates after
+          // Feb 28, 1900 have one extra day added. That is "59" should be
+          // March 1, 1900, but it's considered Feb 29, 1900, which didn't exist.
+          // So all dates greater than 59 have 1 extra day added.
+          // See http://www.cpearson.com/excel/datetime.htm.
+          if ($excel_timestamp >= 59) {
+            $excel_timestamp = $excel_timestamp + 1.0;
           }
-          elseif (preg_match(self::DATE_REGEX_TIME, $value)) {
-            // Only take the time portion of time values.
-            $excel_timestamp = $excel_timestamp - (int) $excel_timestamp;
-            $style_format = 2;
+
+          // Excel does not support dates prior to 0 (Jan 1, 1900). They have to
+          // be represented as plain-text instead.
+          if ($excel_timestamp <= 0) {
+            $output .= '<c r="' . $cell_position . '" t="inlineStr"><is><t>';
+            $output .= htmlspecialchars($value, ENT_QUOTES);
+            $output .= '</t></is></c>';
           }
+          // Now after calculating the Excel "timestamp", save it as a decimal
+          // and point to a style formatter to make it appear as a date/time.
           else {
-            $style_format = 3;
-          }
+            // 1: Dates.
+            // 2: Times.
+            // 3: Date times.
+            // These are tied to style definitions in the styles.xml file
+            // generated by webform_exporter_excel_xlsx::xlsx_parts().
+            if (preg_match(self::DATE_REGEX_DATE, $value)) {
+              $style_format = 1;
+            }
+            elseif (preg_match(self::DATE_REGEX_TIME, $value)) {
+              // Only take the time portion of time values.
+              $excel_timestamp = $excel_timestamp - (int) $excel_timestamp;
+              $style_format = 2;
+            }
+            else {
+              $style_format = 3;
+            }
 
-          $output .= '<c r="' . $cell_position . '" s="' . $style_format . '"><v>';
-          $output .= $excel_timestamp;
-          $output .= '</v></c>';
+            $output .= '<c r="' . $cell_position . '" s="' . $style_format . '"><v>';
+            $output .= $excel_timestamp;
+            $output .= '</v></c>';
+          }
+        }
+        else {
+          $output .= '<c r="' . $cell_position . '" t="inlineStr"' . ($this->wrappable($row_count, $key, $value) ? ' s="4"' : '') . '><is><t>';
+          $output .= htmlspecialchars($value, ENT_QUOTES, 'UTF-8');
+          $output .= '</t></is></c>';
         }
-      }
-      else {
-        $output .= '<c r="' . $cell_position . '" t="inlineStr"' . ($this->wrappable($row_count, $key, $value) ? ' s="4"' : '') . '><is><t>';
-        $output .= htmlspecialchars($value, ENT_QUOTES, 'UTF-8');
-        $output .= '</t></is></c>';
-      }
 
-      // Unbelievably, in PHP you can increment on letters. "Z"++ becomes "AA"
-      // and "AA"++ becomes "AB", identical to Excel column names.
-      $col++;
+        // Unbelievably, in PHP you can increment on letters. "Z"++ becomes "AA"
+        // and "AA"++ becomes "AB", identical to Excel column names.
+        $col++;
+      }
     }
     $output .= '</row>';
     $row++;