Index: ext/misc/series.c
==================================================================
--- ext/misc/series.c
+++ ext/misc/series.c
@@ -329,10 +329,11 @@
   for(i=0; i<pIdxInfo->nConstraint; i++, pConstraint++){
     int iCol;    /* 0 for start, 1 for stop, 2 for step */
     int iMask;   /* bitmask for those column */
     if( pConstraint->iColumn<SERIES_COLUMN_START ) continue;
     iCol = pConstraint->iColumn - SERIES_COLUMN_START;
+    assert( iCol>=0 && iCol<=2 );
     iMask = 1 << iCol;
     if( pConstraint->usable==0 ){
       unusableMask |=  iMask;
       continue;
     }else if( pConstraint->op==SQLITE_INDEX_CONSTRAINT_EQ ){