Error Message: ERROR: UNIQUE constraint matching given keys for referenced table "item" not found

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-- System Information --

Platform:         Windows XP
Version:         5.1
Build:         2600 Service Pack 1

-- Application Information --

Name:         pgAdmin II
Version:         1.6.0
Descripton:

Name:         pgSchema
Version:         1.6.0
Descripton:     PostgreSQL Schema Objects v1.6.0

-- Database Information --

Version:         7.3.2
Descripton:     PostgreSQL 7.3.2 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030213 (Red Hat Linux 8.0 3.2.2-1)

-- Driver Information --

Name:         PostgreSQL
Version:         7.3.100
Descripton:     PostgreSQL 7.3.2 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030213 (Red Hat Linux

-- Error Information --

Description:     ERROR:  UNIQUE constraint matching given keys for referenced
table "item" not found
Number:         -2147467259
Routine:         pgAdmin II:frmTable.cmdOK_Click

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Insert your comment:
Im having trouble creating foreign key for this table (itemsupp) both have
itemcode column but when I created this error occur telling UNIQUE contraint
matching. What shall I do?