Index: src/select.c
==================================================================
--- src/select.c
+++ src/select.c
@@ -4606,11 +4606,12 @@
       nArg = 0;
       regAgg = 0;
     }
     if( pF->iDistinct>=0 ){
       addrNext = sqlite3VdbeMakeLabel(v);
-      assert( nArg==1 );
+      testcase( nArg==0 );  /* Error condition */
+      testcase( nArg>1 );   /* Also an error */
       codeDistinct(pParse, pF->iDistinct, addrNext, 1, regAgg);
     }
     if( pF->pFunc->funcFlags & SQLITE_FUNC_NEEDCOLL ){
       CollSeq *pColl = 0;
       struct ExprList_item *pItem;

Index: test/count.test
==================================================================
--- test/count.test
+++ test/count.test
@@ -189,6 +189,11 @@
   CREATE TABLE t5(a TEXT PRIMARY KEY, b VARCHAR(50)) WITHOUT ROWID;
   INSERT INTO t5 VALUES('bison','jazz');
   SELECT count(*) FROM t5;
 } {1}
 
+do_catchsql_test count-6.1 {
+  CREATE TABLE t6(x);
+  SELECT count(DISTINCT) FROM t6 GROUP BY x;
+} {1 {DISTINCT aggregates must have exactly one argument}}
+
 finish_test