subtraction error in data model

[jvantassel]

I created a full outer join query in power query and loaded it to the data model. I have two columns of values that I need to subtract to get the month to data value for the month.
In the data grid I named the column, and selected the first column and added the minus sign, and then clicked on the second column to get the following formula

Data Act July:=ftbl_GL_Hist_Summary_by_GLAN_full_outer_Data_Act[ftbl_Data_Act.202107]-ftbl_GL_Hist_Summary_by_GLAN_full_outer_Data_Act[ftbl__Data_Act.202106]

The result is "#ERROR" No other information is provided. I thought it was due to nulls, so I replaced the nulls with 0 in Power Query, still no change. HELP!

 

[jvantassel]

I realize that I didn't sum the two columns before subtracting. This is what works.

GL Hist full outer Data Act - July:=SUM(ftbl_GL_Hist_Summary_by_GLAN_full_outer_Data_Act[ftbl_Data_Act.202107])-SUM(ftbl_GL_Hist_Summary_by_GLAN_full_outer_Data_Act[ftbl_Data_Act.202106])