Index: src/alter.c
==================================================================
--- src/alter.c
+++ src/alter.c
@@ -1207,12 +1207,16 @@
   UNUSED_PARAMETER(NotUsed);
   if( zSql==0 ) return;
   if( zTable==0 ) return;
   if( zNew==0 ) return;
   if( iCol<0 ) return;
+  sqlite3BtreeEnterAll(db);
   pTab = sqlite3FindTable(db, zTable, zDb);
-  if( pTab==0 || iCol>=pTab->nCol ) return;
+  if( pTab==0 || iCol>=pTab->nCol ){
+    sqlite3BtreeLeaveAll(db);
+    return;
+  }
   zOld = pTab->aCol[iCol].zName;
   memset(&sCtx, 0, sizeof(sCtx));
   sCtx.iCol = ((iCol==pTab->iPKey) ? -1 : iCol);
 
   /* Parse the SQL statement passed as the first argument. If no error
@@ -1474,10 +1478,11 @@
   renameTokenFree(db, sParse.pRename);
   renameTokenFree(db, sCtx.pList);
   sqlite3DbFree(db, sParse.zErrMsg);
   sqlite3ParserReset(&sParse);
   sqlite3_free(zQuot);
+  sqlite3BtreeLeaveAll(db);
 }
 
 /*
 ** Register built-in functions used to help implement ALTER TABLE
 */