Replacevalue with preceding row value if null

I have a column that is either null or has a value.

What I want is for 'm' to scan the entire column and replace null values with the preceding value.

My Code is this:

= Table.ReplaceValue(#"Added Custom3",each
[MyIndex],each if [MyIndex] = null then
(RawData1[MyIndex]{[Index]-1}) else [MyIndex],Replacer.ReplaceValue,{"MyIndex"} )

But it's not updating, and I don't know why. I'm assuming 'm' updates rows sequentially.