Expecting class path separator ':' before '{' in argument number in Ant + Android build scripts

Today when I tried to Incorporate Ant build scripting with ProGard I got this error.

BUILD FAILED
/path/to/add-proguard-release.xml:35: Expecting class path separator ':' before '{' in argument number 1

to fix this error add following line to your default.properties file:

external.libs.dir=libs