E:\mcp\mcp 1.8.8\customnpcs>gradlew build
This set of MCP mappings was designed for MC 1.8. Use at your own peril.
#################################################
         ForgeGradle 2.1-SNAPSHOT-12e92a7
  https://github.com/MinecraftForge/ForgeGradle
#################################################
               Powered by MCP unknown
             http://modcoderpack.com
         by: Searge, ProfMobius, Fesh0r,
         R4wk, ZeuX, IngisKahn, bspkrs
#################################################
Found AccessTransformer: cnpcs_at.cfg
:deobfCompileDummyTask
:deobfProvidedDummyTask
:compileApiJava
warning: [options] bootstrap class path not set in conjunction with -source 1.6
Note: E:\mcp\mcp 1.8.8\customnpcs\api\noppes\npcs\api\NpcAPI.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 warning
:processApiResources UP-TO-DATE
:apiClasses
:sourceMainJava
:compileJava
warning: [options] bootstrap class path not set in conjunction with -source 1.6
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 warning
:processResources UP-TO-DATE
:classes
:jar
:compileTestJava UP-TO-DATE
:processTestResources UP-TO-DATE
:testClasses UP-TO-DATE
:test UP-TO-DATE
:extractMcpData SKIPPED
:extractMcpMappings SKIPPED
:getVersionJson
:extractUserdev UP-TO-DATE
:genSrgs SKIPPED
:reobfJar
:extractRangemapSrc
:retromapSources
remapping source...
:retromapSources FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':retromapSources'.
> filename net\minecraft\block\NpcBlockHelper.java found package net/minecraft/block->net/minecraft/block but no class map for null

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 2 mins 18.915 secs