Query causesExecSubPlan: NULL value error

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POSTGRESQL BUG REPORT TEMPLATE

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Your name : Richard Smith
Your email address : Rsmith@artschool.com

System Configuration
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Architecture (example: Intel Pentium) :Intel Pentium

Operating System (example: Linux 2.0.26 ELF) :Red Hat Linux 6.2 (Stock)

PostgreSQL version (example: PostgreSQL-6.5.3) :PostgreSQL-6.5.3

Compiler used (example: gcc 2.8.0) :n/a

Please enter a FULL description of your problem:
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I am studying Postgres and have been working thru Bruce Momjian's book.

In figure 8.9 on page 79 I am doing the second query in that
figure. I have typed it in exactly as he presented it but I get a:

"ERROR: ExecSubPlan: null value returned by expression subselect"

type error. I belive my table structures are ok, because the
previous examples (including the one on the same page) have
worked on it.

I emailed this problem to Bruce and he replied:
"That is strange. It sounds like a PostgreSQL bug. My guess is that
there is one or two rows that is causing this. Please report it to the
bugs list so we can fix it."

I have copied and pasted the query and the error message below:
test1=> SELECT f1.firstname, f1.lastname, f1.age FROM friend f1 WHERE
age = (
test1-> SELECT MAX(f2.age) FROM friend f2 WHERE f1.state =
f2.state )
test1-> ORDER BY firstname, lastname ;
ERROR: ExecSubPlan: null value returned by expression subselect
test1=>

I have copied the data set:
test1=> SELECT * FROM friend ;
firstname |lastname |city |state|age
---------------+--------------------+---------------+-----+---
Cindy |Anderson |Denver |CO | 23
Mike |Nichols |Tampa Bay |FL | 20
Mark |Middleton |Indianapolis |IN |
Jack |Burger | | | 27
Dean |Yeager |Plymouth |MA | 24
Ned |Millstone |Cedar Creek |MD | 27
Sandy |Gleason |Ocean City |MA | 33
Victor |Tabor |Williamsport |PA | 22
Dick |Gleason |Ocean City |NJ | 20
Mari-Jo |Anderson |Long Island |NY | 32
Pete |Moxeiter |Saskatoon |SK | 35
Sid |Gleason |Denver |CO | 21
Sam |I Am |New York |NY | 27
Sam |Jackson |Allentown |PA | 23
Deb |Bouchier |Saskatoon |SK | 32
Natalie |Fisher |Fortbywater |NJ | 22
(16 rows)

test1=>

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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This problem occures everytime I run the query.