Oracle case when error: Case not found when Ora-06592 case statement

Yesterday I encountered a very depressing problem. I wrote an oracle function SQL statement as follows:

Code
1 Create   Or   Replace   Function Calcs1 (LF Number )
2 Return   Number   As
3 S1 Number ;
4 K Number ;
5 Begin
6 Case
7 When Lf > 0.92   Then K: = 0 ;
8 When Lf > 0.75   And Lf <= 0.92   Then K: = ( 1.257407 - 2.44079 * Lf + 1.320967 * Lf * Lf );
9 When Lf <= 0.75   Then K: = 0.17 ;
10 Else K: =0; -- This was added later
11 End   Case ;
12 S1: = ( 1 + K * ( 0.92 - Lf) / Lf );
13 -- Dbms_output.put_line ('lf = '| LF | 'K =' | K | 's1 = '| S1 );
14   Return S1;
15 Exception
16 When Invalid_number Then
17 Return   0 ;
18 End ;

After writing, the test passes, but such errors always occur during the call.

I haven't found any problem with case when after reading it for a long time. The problem lies in the column.

The answer http://www.error-code.org.uk/view.asp found here? E = ORACLE-ORA-06592

Case when requires else

I don't understand why the test result is also calculated after I successfully created it, that is, this error occurs during the call.

write it down to avoid making such a mistake in the future.