Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -2518,11 +2518,12 @@
 /*
 ** Estimate the number of rows that will be returned based on
 ** an equality constraint x=VALUE and where that VALUE occurs in
 ** the histogram data.  This only works when x is the left-most
 ** column of an index and sqlite_stat2 histogram data is available
-** for that index.
+** for that index.  When pExpr==NULL that means the constraint is
+** "x IS NULL" instead of "x=VALUE".
 **
 ** Write the estimated row count into *pnRow and return SQLITE_OK. 
 ** If unable to make an estimate, leave *pnRow unchanged and return
 ** non-zero.
 **