What is the correct syntax for counting the number of values in an Excel column if the value is not null?

I tried...
counta('Summary'!B28:B88)

...but got this error:
Couldn't parse Integer field [NUM_GROUPS] with value [counta('Summary'!B28:B88)] --> java.lang.NumberFormatException: For input string: "counta('Summary'!B28:B88)"

Thanks for your help!

Sue