simple makefile

I have 2 dir one is src & include
src contains len.c string.c
include contains len.h
my makefile contains

Code:

vpath %.c src
CPPFLAGS =-I include

string : string.o len.o
	gcc $^ -o $@
string.o: string.c include/len.h
	gcc -c $<
len.o:len.c include/len.h
	gcc -c $<

After giving command make iam getting error

Code:

[root@localhost sting]# make
gcc -c src/string.c
src/string.c:2:16: error: len.h: No such file or directory
make: *** [string.o] Error 1

WHere iam getting wrong