Index: src/os_win.c
==================================================================
--- src/os_win.c
+++ src/os_win.c
@@ -3558,11 +3558,14 @@
   assert( nNewRnd>0 || (flags & SQLITE_MREMAP_EXTEND)==0 );
   if( flags & SQLITE_MREMAP_EXTEND ){
     sqlite3_int64 oldSz;
     rc = winFileSize(id, &oldSz);
     if( rc==SQLITE_OK && nNewRnd>oldSz ){
-      rc = winTruncate(id, nNewRnd);
+      rc = winUnmap(id); /* Cannot truncate the file with an open mapping. */
+      if( rc==SQLITE_OK ){
+        rc = winTruncate(id, nNewRnd);
+      }
     }
     if( rc!=SQLITE_OK ) return rc;
   }
 
   /* If we get this far, unmap any old mapping. */