Problem when filtering on an option field in a page

Hi everyone,

I have an option field in a custom table taking the following values: ,1a,1b,1c,1d,,,,2,,,,3e,3f,3g,3h,3i,,,,,4
When I filter on it in the corresponding page, I get strange results:

Filter on "1a" --> correct results
Filter on "1b" --> entries with "2"
Filter on "1c" --> correct results
Filter on "1d" --> entries with "4"
Filter on "2" --> correct results
Filter on "3e" --> correct results
Filter on "3f" --> correct results
Filter on "3g" --> correct results
Filter on "3h" --> correct results
Filter on "3i" --> correct results
Filter on "4"  --> error message : '21' is not an option.The existing options are ,1a,1b,1c,1d,2,3e,3f,3g,3h,3i,4

(21 is the index of "4" in the option string)

I can't understand why it is working for some values and not for the rest...Any ideas?

Thanks in advance