Short question: What am I doing wrong?

I have a range of cells:

The range of cells (C22:ap34) is populated by the formula:
=INDIRECT(C$4&"!E"&$A22) thru =INDIRECT(AP$4&"!E"&$A34) - this created
a summary table pulling the values from other sheets as indicated by
the column header, always found in row 4.

In cell C20, I have the formula: =MAX(C22:AP34) - which shows me the
max value in that range.

In a cell, adjacent to the cell showing the max value, I want to show
the row and column header that matches that maximum value.

I have entered and INDEX/MATCH formula, but it will only result in a
"#N/A" error. I have tried it using the actual number instead of the
cell reference (C20) to the max value. I have tried to enter the match
formula on its own. I am using the ,0 parameter for an exact match, but
the same "#N/A" appears if I change it to ,1 or ,-1. Here's that exact
formula:

=INDEX(C21:AP21,MATCH(C20,C22:AP34,0))

The column headers are repeated on C21:AP21.

What am I doing wrong? Is the range too large - is it the formula that
populates the range?