Referencing a blank field in TM

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Has anyone worked with an IF_THEN_ELSE statement on a calculated attribute in transaction matching?  I'm trying to say IF attribute A is blank  THEN select attribute B ELSE choose attribute A.  I thought I could use = ' ' but that doesn't appear to be working.

My formula looks something like this.

IF_THEN_ELSE( {Attribute A} = ' ' ,SUBSTRING({Attribute B},1,10), {Attribute A} )