Index: src/parse.y
==================================================================
--- src/parse.y
+++ src/parse.y
@@ -959,11 +959,11 @@
 %include {
   /* A routine to convert a binary TK_IS or TK_ISNOT expression into a
   ** unary TK_ISNULL or TK_NOTNULL expression. */
   static void binaryToUnaryIfNull(Parse *pParse, Expr *pY, Expr *pA, int op){
     sqlite3 *db = pParse->db;
-    if( db->mallocFailed==0 && pY->op==TK_NULL ){
+    if( pY && pA && pY->op==TK_NULL ){
       pA->op = (u8)op;
       sqlite3ExprDelete(db, pA->pRight);
       pA->pRight = 0;
     }
   }

Index: test/expr.test
==================================================================
--- test/expr.test
+++ test/expr.test
@@ -203,10 +203,14 @@
 test_expr expr-1.125 {i1=6, i2=NULL} \
   {CASE WHEN i1 IS NOT i2 THEN 'yes' ELSE 'no' END} yes
 test_expr expr-1.126 {i1=8, i2=8} \
   {CASE WHEN i1 IS NOT i2 THEN 'yes' ELSE 'no' END} no
 
+do_catchsql_test expr-1.127 {
+  SELECT 1 IS #1;
+} {1 {near "#1": syntax error}}
+
 ifcapable floatingpoint {if {[working_64bit_int]} {
   test_expr expr-1.200\
       {i1=9223372036854775806, i2=1} {i1+i2}      9223372036854775807
   test_realnum_expr expr-1.201\
       {i1=9223372036854775806, i2=2} {i1+i2}      9.22337203685478e+18