When trying to "/summon EntityFluidCow" the following error happens:

[08/4/2015 12:49:31 PM] net.minecraft.util.ReportedException: Saving entity NBT
[08/4/2015 12:49:31 PM]     at net.minecraft.entity.Entity.func_70109_d(Entity.java:1359) ~[sa.class:?]
[08/4/2015 12:49:31 PM]     at net.minecraft.entity.Entity.func_70039_c(Entity.java:1298) ~[sa.class:?]
[08/4/2015 12:49:31 PM]     at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75820_a(AnvilChunkLoader.java:346) [aqk.class:?]
[08/4/2015 12:49:31 PM]     at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75816_a(AnvilChunkLoader.java:197) [aqk.class:?]
[08/4/2015 12:49:31 PM]     at net.minecraft.world.gen.ChunkProviderServer.func_73242_b(ChunkProviderServer.java:256) [ms.class:?]
[08/4/2015 12:49:31 PM]     at net.minecraft.world.gen.ChunkProviderServer.func_73151_a(ChunkProviderServer.java:302) [ms.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.world.WorldServer.func_73044_a(WorldServer.java:809) [mt.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.server.MinecraftServer.func_71267_a(MinecraftServer.java:318) [MinecraftServer.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:569) [MinecraftServer.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:111) [bsx.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [MinecraftServer.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [?:?]
[08/4/2015 12:49:32 PM] Caused by: java.lang.NullPointerException
[08/4/2015 12:49:32 PM]     at com.robrit.moofluids.common.entity.EntityFluidCow.func_70014_b(EntityFluidCow.java:313) ~[EntityFluidCow.class:?]
[08/4/2015 12:49:32 PM]     at net.minecraft.entity.Entity.func_70109_d(Entity.java:1342) ~[sa.class:?]
[08/4/2015 12:49:32 PM]     ... 11 more

I suspect the /summon command doesn't actually set a random fluid and when MC tries to save the state of the cow, it balks.

I have the same issue on cow spawning while i'm going around, usually in new chunks. I not used commands to spawn the cows

MooFluids-1.12.2-1.7.11.02a.jar

[15:58:07] [Server thread/ERROR]: An Entity type com.robrit.moofluids.common.entity.EntityFluidCow has thrown an exception trying to write state. It will not persist. Report this to the mod author
net.minecraft.util.ReportedException: Saving entity NBT
at net.minecraft.entity.Entity.func_189511_e(Entity.java:1772) ~[vg.class:?]
at net.minecraft.entity.Entity.func_70039_c(Entity.java:1654) ~[vg.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75820_a(AnvilChunkLoader.java:361) [aye.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75816_a(AnvilChunkLoader.java:173) [aye.class:?]
at net.minecraft.world.gen.ChunkProviderServer.func_73242_b(ChunkProviderServer.java:202) [on.class:?]
at net.minecraft.world.gen.ChunkProviderServer.func_186027_a(ChunkProviderServer.java:230) [on.class:?]
at net.minecraft.world.WorldServer.func_73044_a(WorldServer.java:1007) [oo.class:?]
at net.minecraft.server.MinecraftServer.func_71267_a(MinecraftServer.java:408) [MinecraftServer.class:?]
at net.minecraft.server.integrated.IntegratedServer.func_71267_a(IntegratedServer.java:252) [chd.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:691) [MinecraftServer.class:?]
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:185) [chd.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]
Caused by: java.lang.NullPointerException
at com.robrit.moofluids.common.entity.EntityFluidCow.func_70014_b(EntityFluidCow.java:327) ~[EntityFluidCow.class:?]
at net.minecraft.entity.Entity.func_189511_e(Entity.java:1743) ~[vg.class:?]
... 12 more