Error trying to compile project. Can not merge new index 65549 into a non-jumbo instruction!

Would anyone know to tell me what's going on and how to solve it? I'm using Android Studio. The following is a complete list of the error.

UNEXPECTED TOP-LEVEL EXCEPTION:
com.android.dex.DexIndexOverflowException: Cannot merge new index 65549 into a non-jumbo instruction!
    at com.android.dx.merge.InstructionTransformer.jumboCheck(InstructionTransformer.java:109)
    at com.android.dx.merge.InstructionTransformer.access$800(InstructionTransformer.java:26)
    at com.android.dx.merge.InstructionTransformer$StringVisitor.visit(InstructionTransformer.java:72)
    at com.android.dx.io.CodeReader.callVisit(CodeReader.java:114)
    at com.android.dx.io.CodeReader.visitAll(CodeReader.java:89)
    at com.android.dx.merge.InstructionTransformer.transform(InstructionTransformer.java:49)
    at com.android.dx.merge.DexMerger.transformCode(DexMerger.java:842)
    at com.android.dx.merge.DexMerger.transformMethods(DexMerger.java:813)
    at com.android.dx.merge.DexMerger.transformClassData(DexMerger.java:785)
    at com.android.dx.merge.DexMerger.transformClassDef(DexMerger.java:682)
    at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:542)
    at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:171)
    at com.android.dx.merge.DexMerger.merge(DexMerger.java:189)
    at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:454)
    at com.android.dx.command.dexer.Main.runMonoDex(Main.java:303)
    at com.android.dx.command.dexer.Main.run(Main.java:246)
    at com.android.dx.command.dexer.Main.main(Main.java:215)
    at com.android.dx.command.Main.main(Main.java:106)

 FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:dexDebug'.
> com.android.ide.common.internal.LoggedErrorException: Failed to run command:
    /Users/Oliver/Library/Android/sdk/build-tools/21.1.2/dx --dex --output /Users/Oliver/AndroidStudioProjects/TravelExpenseControl/app/build/intermediates/dex/debug --input-list=/Users/Oliver/AndroidStudioProjects/TravelExpenseControl/app/build/intermediates/tmp/dex/debug/inputList.txt
  Error Code:
    2
  Output:

    UNEXPECTED TOP-LEVEL EXCEPTION:
    com.android.dex.DexIndexOverflowException: Cannot merge new index 65549 into a non-jumbo instruction!
        at com.android.dx.merge.InstructionTransformer.jumboCheck(InstructionTransformer.java:109)
        at com.android.dx.merge.InstructionTransformer.access$800(InstructionTransformer.java:26)
        at com.android.dx.merge.InstructionTransformer$StringVisitor.visit(InstructionTransformer.java:72)
        at com.android.dx.io.CodeReader.callVisit(CodeReader.java:114)
        at com.android.dx.io.CodeReader.visitAll(CodeReader.java:89)
        at com.android.dx.merge.InstructionTransformer.transform(InstructionTransformer.java:49)
        at com.android.dx.merge.DexMerger.transformCode(DexMerger.java:842)
        at com.android.dx.merge.DexMerger.transformMethods(DexMerger.java:813)
        at com.android.dx.merge.DexMerger.transformClassData(DexMerger.java:785)
        at com.android.dx.merge.DexMerger.transformClassDef(DexMerger.java:682)
        at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:542)
        at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:171)
        at com.android.dx.merge.DexMerger.merge(DexMerger.java:189)
        at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:454)
        at com.android.dx.command.dexer.Main.runMonoDex(Main.java:303)
        at com.android.dx.command.dexer.Main.run(Main.java:246)
        at com.android.dx.command.dexer.Main.main(Main.java:215)
        at com.android.dx.command.Main.main(Main.java:106)