Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -932,11 +932,10 @@
 ** caller to eventually free p->idxStr if p->needToFreeIdxStr indicates
 ** that this is required.
 */
 static int vtabBestIndex(Parse *pParse, Table *pTab, sqlite3_index_info *p){
   sqlite3_vtab *pVtab = sqlite3GetVTable(pParse->db, pTab)->pVtab;
-  int i;
   int rc;
 
   TRACE_IDX_INPUTS(p);
   rc = pVtab->pModule->xBestIndex(pVtab, p);
   TRACE_IDX_OUTPUTS(p);
@@ -951,16 +950,20 @@
     }
   }
   sqlite3_free(pVtab->zErrMsg);
   pVtab->zErrMsg = 0;
 
+#if 0
+  /* This error is now caught by the caller.
+  ** Search for "xBestIndex malfunction" below */
   for(i=0; i<p->nConstraint; i++){
     if( !p->aConstraint[i].usable && p->aConstraintUsage[i].argvIndex>0 ){
       sqlite3ErrorMsg(pParse, 
           "table %s: xBestIndex returned an invalid plan", pTab->zName);
     }
   }
+#endif
 
   return pParse->nErr;
 }
 #endif /* !defined(SQLITE_OMIT_VIRTUALTABLE) */
 
@@ -2833,13 +2836,14 @@
       int j = pIdxCons->iTermOffset;
       if( iTerm>=nConstraint
        || j<0
        || j>=pWC->nTerm
        || pNew->aLTerm[iTerm]!=0
+       || pIdxCons->usable==0
       ){
         rc = SQLITE_ERROR;
-        sqlite3ErrorMsg(pParse,"%s.xBestIndex() malfunction",pSrc->pTab->zName);
+        sqlite3ErrorMsg(pParse,"%s.xBestIndex malfunction",pSrc->pTab->zName);
         return rc;
       }
       testcase( iTerm==nConstraint-1 );
       testcase( j==0 );
       testcase( j==pWC->nTerm-1 );
@@ -2857,11 +2861,11 @@
         ** is not necessarily related to the order of output terms and
         ** (2) Multiple outputs from a single IN value will not merge
         ** together.  */
         pIdxInfo->orderByConsumed = 0;
         pIdxInfo->idxFlags &= ~SQLITE_INDEX_SCAN_UNIQUE;
-        *pbIn = 1;
+        *pbIn = 1; assert( (mExclude & WO_IN)==0 );
       }
     }
   }
 
   pNew->nLTerm = mxTerm+1;
@@ -2970,14 +2974,15 @@
 
     /* If the plan produced by the earlier call uses an IN(...) term, call
     ** xBestIndex again, this time with IN(...) terms disabled. */
     if( rc==SQLITE_OK && bIn ){
       rc = whereLoopAddVirtualOne(pBuilder, mPrereq, ALLBITS, WO_IN, p, &bIn);
+      assert( bIn==0 );
       mBestNoIn = pNew->prereq & ~mPrereq;
       if( mBestNoIn==0 ){
         seenZero = 1;
-        if( bIn==0 ) seenZeroNoIN = 1;
+        seenZeroNoIN = 1;
       }
     }
 
     /* Call xBestIndex once for each distinct value of (prereqRight & ~mPrereq) 
     ** in the set of terms that apply to the current virtual table.  */

Index: test/vtab6.test
==================================================================
--- test/vtab6.test
+++ test/vtab6.test
@@ -564,15 +564,15 @@
 
 do_test vtab6-11.4.1 {
   catchsql {
     SELECT a, b, c FROM ab NATURAL JOIN bc;
   }
-} {1 {table ab: xBestIndex returned an invalid plan}}
+} {1 {ab.xBestIndex malfunction}}
 do_test vtab6-11.4.2 {
   catchsql {
     SELECT a, b, c FROM bc NATURAL JOIN ab;
   }
-} {1 {table bc: xBestIndex returned an invalid plan}}
+} {1 {bc.xBestIndex malfunction}}
 
 unset ::echo_module_ignore_usable
 
 finish_test