--- src/vdbe.c
+++ src/vdbe.c
@@ -5323,17 +5323,17 @@
 
 #ifndef SQLITE_OMIT_WAL
   zFilename = sqlite3PagerFilename(pPager);
 
   /* Do not allow a transition to journal_mode=WAL for a database
   ** in temporary storage or if the VFS does not support shared memory 
   */
   if( eNew==PAGER_JOURNALMODE_WAL
-   && (zFilename[0]==0                         /* Temp file */
+   && (zFilename==NULL || zFilename[0]==0      /* Temp file */
        || !sqlite3PagerWalSupported(pPager))   /* No shared-memory support */
   ){
     eNew = eOld;
   }
 
   if( (eNew!=eOld)
    && (eOld==PAGER_JOURNALMODE_WAL || eNew==PAGER_JOURNALMODE_WAL)
   ){