A singular expression is required

This error message can result from trying to compare two lists or a list to an object. In general, all comparisons must be made between two singular objects. For instance:

Q: Versions of files of folder "c:\temp2" = versions of files of folder "c:\temp"
e: A singular expression is required.

Returns the error because comparing two lists is undefined. This will result in an error even if both folders contain exactly the same files. Similarly,

Versions of files of folder "c:\temp" = "4.5"

Gives the same error because you can’t compare a list to a single value. You will get this error even if there is only one file in the folder "c:\temp" whose version is 4.5. If you want to iterate through multiple comparisons, use the whose statement:

versions of files whose (version of it = "4.5") of folder "c:\temp"

You can also get this error when you try to operate on two objects with different plurality, as in this example where we attempt to add a number and a list:

Q: 1 + (2;3)
E: A singular expression is required.