Zend certified PHP/Magento developer

Ref error with indirect

I am trying to use INDIRECT to dynamically reference another spreadsheet, but I am getting a ref error. The formula that I am using is =INDIRECT("'"&B3&"["&B4&"]Assignment Grades'!E10").

When I take the input to INDIRECT and throw an equals sign in front of it, I get this as the output: 'c:usersahmeedownloads[MATH_110_Homework_1_scores.xlsx]Assignment Grades'!E10, which looks right to me. Further, I copied this output to another cell and put an equals sign in front, and it returned an actual value. To be clear, the formula ='c:usersahmeedownloads[MATH_110_Homework_1_scores.xlsx]Assignment Grades'!E10 returns a number.

I’m really having trouble seeing where things are breaking down here, and any help would be appreciated.