I've seen this one too here, and what I found out is the following:
This is caused by the fact that 2 months ago a bug was introduced which didn't save type information for stored procedure parameters correctly.
Refresh the catalog and it should be over. You'll get also an exception now if you edit the stored proc's properties.
THe bug was quickly fixed but it seemed some projects which were refreshed/created with that version are still carrying along the empty type information for the stored proc parameters.
If refreshing the catalog / regeneration of the code still gives the same results, it's something else, however it looks very familiar.