Index: src/os_win.c
==================================================================
--- src/os_win.c
+++ src/os_win.c
@@ -3883,10 +3883,11 @@
     if( !osReadFile(pFile->h, &notUsed1, 1, &notUsed2, pOverlapped) ){
       lastErrno = osGetLastError();
       if( winIsLockConflict(lastErrno) ){
         if( pLockType ) *pLockType = WINSHM_WRLCK;
       }else if( winIsLockMissing(lastErrno) ){
+        assert( bReadOnly );
         if( pLockType ) *pLockType = WINSHM_UNLCK;
       }else{
         return SQLITE_IOERR_READ;
       }
     }else{