patsubst strange behavior when substituting the empty string

I recently observed a strange `patsubst` behavior when substituting the empty string:

```
$ cat Makefile

.PHONY: all
all:;
$(info $(patsubst %.,,1.2))

$ make
1.2
make: Nothing to be done for 'all'.
```

I was expecting:

```
$ make
2
make: Nothing to be done for 'all'.
```

I read the manual twice but I cannot explain this behavior. What did I miss?
-- 
Renaud Pacalet
Télécom Paris
Campus SophiaTech
450 Route des Chappes, CS 50193
06904 Biot Sophia Antipolis cedex, FRANCE
Tel : +33 (0) 4 9300 8402
Web : http://www.telecom-paris.fr/