diff --git a/source/workbook/workbook.cpp b/source/workbook/workbook.cpp
index fcea0b12..59f9a8af 100644
--- a/source/workbook/workbook.cpp
+++ b/source/workbook/workbook.cpp
@@ -1374,9 +1374,18 @@ std::size_t workbook::add_shared_string(const rich_text &shared, bool allow_dupl
         }
     }
 
-    auto sz = d_->shared_strings_ids_.size();
-    d_->shared_strings_ids_[shared] = sz;
+ 	//it can happen that similar strings are more then onetime in the shared stringtable (Excel bugfix?)
+	//shared_strings_values map should start on position 0
+	auto sz = d_->shared_strings_values_.size();
+    if (d_->shared_strings_values_.count(sz) > 0)
+    {
+        // something went wrong!
+        throw invalid_file("Error in shared string table!");
+	}
+
     d_->shared_strings_values_[sz] = shared;
+    d_->shared_strings_ids_[shared] = sz;
+
 
     return sz;
 }