field calculator - Debugging ERROR 000539 from CalculateField in ArcPy?

I previously successfully used the sample code here: How to calculate Field using part of filename in ModelBuilder?

# Import standard library modules
import arcpy, os, sys

from arcpy import env

# Allow for file overwrite
arcpy.env.overwriteOutput = True

# Set the workspace directory 
env.workspace = r"C:\temp.gdb" 

# Get the list of the featureclasses to process
fc_tables = arcpy.ListFeatureClasses()


# Loop through each file and perform the processing
for fc in fc_tables:
    print str("processing " + fc)

    # Define field name and expression
    field = "SID"
    expression = str(fc[:5]) #subsets first 5 characters of fc name

    # Create a new field with a new name

    arcpy.AddField_management(fc,field,"TEXT")

    # Calculate field here
    arcpy.CalculateField_management(fc, field, "expression", "PYTHON") 

But now I'm running it again, with no changes, even with the same datasets I used before, and it no longer works. I cannot see how or why it errors out.

Error message:

Runtime error : ERROR 000539: Error running expression: expression : name 'expression' is not defined Failed to execute (CalculateField).