diff --git a/src/main.h b/src/main.h
index 423891c3..262e77e8 100644
--- a/src/main.h
+++ b/src/main.h
@@ -944,9 +944,10 @@ public:
         fileout << FLATDATA(pchMessageStart) << nSize;
 
         // Write block
-        nBlockPosRet = ftell(fileout);
-        if (nBlockPosRet == -1)
+        long fileOutPos = ftell(fileout);
+        if (fileOutPos < 0)
             return error("CBlock::WriteToDisk() : ftell failed");
+        nBlockPosRet = fileOutPos;
         fileout << *this;
 
         // Flush stdio buffers and commit to disk before returning