diff --git a/src/client/mapio.cpp b/src/client/mapio.cpp
index e6780467..50bae75e 100644
--- a/src/client/mapio.cpp
+++ b/src/client/mapio.cpp
@@ -35,16 +35,20 @@
 void Map::loadOtbm(const std::string& fileName)
 {
     try {
+        if(!g_things.isOtbLoaded())
+            stdext::throw_exception("OTB isn't loaded yet to load a map.");
+
         FileStreamPtr fin = g_resources.openFile(fileName);
         if(!fin)
             stdext::throw_exception(stdext::format("Unable to load map '%s'", fileName));
-
         fin->cache();
-        if(!g_things.isOtbLoaded())
-            stdext::throw_exception("OTB isn't loaded yet to load a map.");
 
-        if(fin->getU32())
-            stdext::throw_exception("Unknown file version detected");
+        char identifier[4];
+        if(fin->read(identifier, 1, 4) < 4)
+            stdext::throw_exception("Could not read file identifier");
+
+        if(memcmp(identifier, "OTBM", 4) != 0 && memcmp(identifier, "\0\0\0\0", 4) != 0)
+            stdext::throw_exception(stdext::format("Invalid file identifier detected: %s", identifier));
 
         BinaryTreePtr root = fin->getBinaryTree();
         if(root->getU8())