SAS Error: Null parameters for SUM are invalid

So I have a data named table1 as follows:

Obs  ID  M_201812  M_201901      M_201902    M_201903

1    X1     1         .             .           . 
2    X2     1         1             .           . 
3    X3     .         1             1           . 
4    X4     .         1             .           . 
5    X5     .         1             .           . 
6    X6     1         .             .           . 
7    X7     1         1             .           . 
8    X8     1         1             .           . 
9    X9     .         .             1           . 
10   X10    1         1             .           . 

Each column here is a month, which is dynamically generated based on some previously run macro. The months will be dynamic and will vary. What I need to do is calculate sums of last 3 months, last 6 months and last 12 months. The approach I had in my mind was as follows: A) Store the column names in a macro variable:

proc sql noprint;
    select distinct name
    into :cols2 separated by ','  
    from dictionary.columns
    where upcase(memname) = 'Table1' and name not in ('ID');
    ;
quit;
%put &cols2.

The output was as follows:

M_201812,M_201901,M_201902,M_201903

B) Create sums thereafter based on the number of items in the variable:

data table1;
set table1;

if count("&cols2",",") <=3 then do;
3m_total=sum(of &cols2);
6m_total=sum(of &cols2);
12m_total=sum(of &cols2);
end;
else if 3< count("&cols2",",") <=6 then do;
3m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)));
6m_total=sum(of &cols2);
12m_total=sum(of &cols2);
end;
else if 6< count("&cols2",",") <=12 then do;
3m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)));
6m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)),%scan(%superQ(cols2),-4,%str(,)),%scan(%superQ(cols2),-5,%str(,)),%scan(%superQ(cols2),-6,%str(,)));
12m_total=sum(of &cols2);
else do;
    3m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)));
    6m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)),%scan(%superQ(cols2),-4,%str(,)),%scan(%superQ(cols2),-5,%str(,)),%scan(%superQ(cols2),-6,%str(,)));
    12m_total=sum(%scan(%superQ(cols2),-1,%str(,)),%scan(%superQ(cols2),-2,%str(,)),%scan(%superQ(cols2),-3,%str(,)),%scan(%superQ(cols2),-4,%str(,)),%scan(%superQ(cols2),-5,%str(,)),%scan(%superQ(cols2),-6,%str(,)),
    %scan(%superQ(cols2),-7,%str(,)),%scan(%superQ(cols2),-8,%str(,)),%scan(%superQ(cols2),-9,%str(,)),%scan(%superQ(cols2),-10,%str(,)),%scan(%superQ(cols2),-11,%str(,)),%scan(%superQ(cols2),-12,%str(,)));
    end;
    run;

Basically we get 12 months sum only if there are 12 monthly columns available. If only 3 months are available, then 3months sum=6months sum=12months sum. After running the code, I get the following error:

ERROR 159-185: Null parameters for SUM are invalid.

This happens at the last else do statement. I can't for the life of me figure out why won't sas be able to read a simple if-then-do-else statement. Is there an error in the if conditions or in calling the macro variable? Any help here would be appreciated. Thanks a lot.