App crashes on API Level below 21, which is caused by SQLite classes

I am getting exception NoClassDefFoundError whenever I try to run the app on Android API Level below 21.

07-23 03:52:53.410  11681-11681/com.xyz.abc E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.xyz.abc, PID: 11681
    java.lang.NoClassDefFoundError: com.xyz.abc.database.DB$DBHelper
            at com.xyz.abc.database.DB.<init>(DB.java:27)
            at com.xyz.abc.abcApp.onCreate(abcApp.java:39)
            at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1007)
            at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4328)
            at android.app.ActivityThread.access$1500(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

This is my gradle script

compileSdkVersion 21
buildToolsVersion "21.1.1"

defaultConfig {
    applicationId "com.xyz.abc"
    minSdkVersion 15
    targetSdkVersion 21
    versionCode 1
    versionName "1.0"
    multiDexEnabled true
}

This is my first Android App, I think I've made some silly mistake somewhere. Please help me out