diff --git a/src/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactory.php b/src/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactory.php
index 8fffdb0..36e0bfe 100644
--- a/src/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactory.php
+++ b/src/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactory.php
@@ -78,7 +78,7 @@ class CachingStrategyFactory
      * Returns the best caching strategy, given the number of unique shared strings
      * and the amount of memory available.
      *
-     * @param int $sharedStringsUniqueCount Number of unique shared strings
+     * @param int|null $sharedStringsUniqueCount Number of unique shared strings (NULL if unknown)
      * @param string|void $tempFolder Temporary folder where the temporary files to store shared strings will be stored
      * @return CachingStrategyInterface The best caching strategy
      */
@@ -95,11 +95,16 @@ class CachingStrategyFactory
      * Returns whether it is safe to use in-memory caching, given the number of unique shared strings
      * and the amount of memory available.
      *
-     * @param int $sharedStringsUniqueCount Number of unique shared strings
+     * @param int|null $sharedStringsUniqueCount Number of unique shared strings (NULL if unknown)
      * @return bool
      */
     protected function isInMemoryStrategyUsageSafe($sharedStringsUniqueCount)
     {
+        // if the number of shared strings in unknown, do not use "in memory" strategy
+        if ($sharedStringsUniqueCount === null) {
+            return false;
+        }
+
         $memoryAvailable = $this->getMemoryLimitInKB();
 
         if ($memoryAvailable === -1) {
diff --git a/src/Spout/Reader/XLSX/Helper/SharedStringsHelper.php b/src/Spout/Reader/XLSX/Helper/SharedStringsHelper.php
index 350024a..0f41e90 100644
--- a/src/Spout/Reader/XLSX/Helper/SharedStringsHelper.php
+++ b/src/Spout/Reader/XLSX/Helper/SharedStringsHelper.php
@@ -147,7 +147,7 @@ class SharedStringsHelper
      * Returns the shared strings unique count, as specified in <sst> tag.
      *
      * @param \Box\Spout\Reader\Wrapper\XMLReader $xmlReader XMLReader instance
-     * @return int Number of unique shared strings in the sharedStrings.xml file
+     * @return int|null Number of unique shared strings in the sharedStrings.xml file
      * @throws \Box\Spout\Common\Exception\IOException If sharedStrings.xml is invalid and can't be read
      */
     protected function getSharedStringsUniqueCount($xmlReader)
@@ -167,13 +167,13 @@ class SharedStringsHelper
             $uniqueCount = $xmlReader->getAttribute('count');
         }
 
-        return intval($uniqueCount);
+        return ($uniqueCount !== null) ? intval($uniqueCount) : null;
     }
 
     /**
      * Returns the best shared strings caching strategy.
      *
-     * @param int $sharedStringsUniqueCount
+     * @param int|null $sharedStringsUniqueCount Number of unique shared strings (NULL if unknown)
      * @return CachingStrategyInterface
      */
     protected function getBestSharedStringsCachingStrategy($sharedStringsUniqueCount)
diff --git a/tests/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactoryTest.php b/tests/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactoryTest.php
index ea77b4f..e61760a 100644
--- a/tests/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactoryTest.php
+++ b/tests/Spout/Reader/XLSX/Helper/SharedStringsCaching/CachingStrategyFactoryTest.php
@@ -15,6 +15,7 @@ class CachingStrategyFactoryTest extends \PHPUnit_Framework_TestCase
     public function dataProviderForTestGetBestCachingStrategy()
     {
         return [
+            [null, -1, 'FileBasedStrategy'],
             [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE, -1, 'FileBasedStrategy'],
             [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE + 10, -1, 'FileBasedStrategy'],
             [CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE - 10, -1, 'InMemoryStrategy'],
@@ -27,7 +28,7 @@ class CachingStrategyFactoryTest extends \PHPUnit_Framework_TestCase
     /**
      * @dataProvider dataProviderForTestGetBestCachingStrategy
      *
-     * @param int $sharedStringsUniqueCount
+     * @param int|null $sharedStringsUniqueCount
      * @param int $memoryLimitInKB
      * @param string $expectedStrategyClassName
      * @return void
diff --git a/tests/Spout/Reader/XLSX/ReaderTest.php b/tests/Spout/Reader/XLSX/ReaderTest.php
index 3fa6ec1..02c91ed 100644
--- a/tests/Spout/Reader/XLSX/ReaderTest.php
+++ b/tests/Spout/Reader/XLSX/ReaderTest.php
@@ -126,6 +126,20 @@ class ReaderTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportSheetWithSharedStringsMissingUniqueCountAndCountAttributes()
+    {
+        $allRows = $this->getAllRowsForFile('one_sheet_with_shared_strings_missing_unique_count_and_count.xlsx');
+
+        $expectedRows = [
+            ['s1--A1', 's1--B1'],
+            ['s1--A2', 's1--B2'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */
diff --git a/tests/resources/xlsx/one_sheet_with_shared_strings_missing_unique_count_and_count.xlsx b/tests/resources/xlsx/one_sheet_with_shared_strings_missing_unique_count_and_count.xlsx
new file mode 100644
index 0000000..6ef7ffd
Binary files /dev/null and b/tests/resources/xlsx/one_sheet_with_shared_strings_missing_unique_count_and_count.xlsx differ