Error getting spatial reference as string in arcpy

I am trying to get the string associated with a spatial reference to print into an xml using arcpy and xml.cElementTree. Using the snippet:

...
spatial_ref = arcpy.GetParameterAsText(3)
...
Prj = et.SubElement(spaceref, "Prj")
space = arcpy.SpatialReference(spatial_ref)
Prj.text = arcpy.SpatialReference.exportToString(space)
...

Using essentially the same method, I am able to get the string to print with IDLE by pointing directly to a projection (.prj) file, e.g.:

u"GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]];-400 -400 1000000000;-100000 10000;-100000 10000;8.98315284119522E-09;0.001;0.001;IsHighPrecision"

but Arc returns this error when I try to run my script:

Traceback (most recent call last): File "\DEFOE\Proc Storage\ORTHO\Arc Custom Tools\ImportUltracam.py", line 99, in space = arcpy.SpatialReference(spatial_ref) File "c:\program files (x86)\arcgis\desktop10.2\arcpy\arcpy\arcobjects\mixins.py", line 927, in init self._arc_object.createFromFile(item) RuntimeError: ERROR 999999: Error executing function.

Is there a way to input a spatial reference with Arc and return the string to be printed into the xml?

Update Playing with IDLE, I tried with an EPSG code and was able to do exactly what I wanted:

>>> epsg = arcpy.SpatialReference(32618)
>>> print epsg
<geoprocessing spatial reference object object at 0x000000000C7D0710>
>>> arcpy.SpatialReference.exportToString(epsg)
u"PROJCS['WGS_1984_UTM_Zone_18N',GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]],PROJECTION['Transverse_Mercator'],PARAMETER['False_Easting',500000.0],PARAMETER['False_Northing',0.0],PARAMETER['Central_Meridian',-75.0],PARAMETER['Scale_Factor',0.9996],PARAMETER['Latitude_Of_Origin',0.0],UNIT['Meter',1.0]];-5120900 -9998100 10000;-100000 10000;-100000 10000;0.001;0.001;0.001;IsHighPrecision"

So I tried with an EPSG code in the script:

spatial_ref = "32618"
Prj = et.SubElement(spaceref, "Prj")
space = arcpy.SpatialReference(spatial_ref)
Prj.text = arcpy.SpatialReference.exportToString(space)

And I get the same error.