ORA-01722: invalid number 

Cause: The attempted conversion of a character string to a number failed because 
the character string was not a valid numeric literal. Only numeric fields or character 
fields containing numeric data may be used in arithmetic functions or expressions. 
Only numeric fields may be added to or subtracted from dates. 

Action: Check the character strings in the function or expression. 
Check that they contain only numbers, a sign, a decimal point, and the 
character "E" or "e" and retry the operation.  

  PROCEDURE ASSIGN_LEAF(P_SYS_ID_NUM NUMBER,
                        P_NEW_PARENT NUMBER,
                        P_LEAF_NODE  NUMBER,
                        P_EXCEPTION  NUMBER);

          ASSIGN_LEAF(HIER_REC.SYS_ID_NUM,
                        NODE_REC.ROLLUP_NODE,
                        LEAF_REC.LEAF_NODE,
                        0);

          ASSIGN_LEAF(  TO_NUMBER(HIER_REC.SYS_ID_NUM),
                        TO_NUMBER(NODE_REC.ROLLUP_NODE),
                        TO_NUMBER(LEAF_REC.LEAF_NODE),
                        0);