diff --git a/fabric-object-builder-api-v1/src/main/java/net/fabricmc/fabric/mixin/object/builder/BlockEntityTypeMixin.java b/fabric-object-builder-api-v1/src/main/java/net/fabricmc/fabric/mixin/object/builder/BlockEntityTypeMixin.java
index ea2ae7a82..2d2b0df7b 100644
--- a/fabric-object-builder-api-v1/src/main/java/net/fabricmc/fabric/mixin/object/builder/BlockEntityTypeMixin.java
+++ b/fabric-object-builder-api-v1/src/main/java/net/fabricmc/fabric/mixin/object/builder/BlockEntityTypeMixin.java
@@ -16,11 +16,12 @@
 
 package net.fabricmc.fabric.mixin.object.builder;
 
-import java.util.HashSet;
-import java.util.Objects;
-import java.util.Set;
-
+import com.google.common.collect.ImmutableSet;
 import com.mojang.datafixers.types.Type;
+import net.fabricmc.fabric.api.object.builder.v1.block.entity.FabricBlockEntityType;
+import net.minecraft.world.level.block.Block;
+import net.minecraft.world.level.block.entity.BlockEntity;
+import net.minecraft.world.level.block.entity.BlockEntityType;
 import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Mutable;
@@ -28,10 +29,10 @@
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
-import net.fabricmc.fabric.api.object.builder.v1.block.entity.FabricBlockEntityType;
-import net.minecraft.world.level.block.Block;
-import net.minecraft.world.level.block.entity.BlockEntity;
-import net.minecraft.world.level.block.entity.BlockEntityType;
+
+import java.util.HashSet;
+import java.util.Objects;
+import java.util.Set;
 
 @Mixin(BlockEntityType.class)
 public class BlockEntityTypeMixin<T extends BlockEntity> implements FabricBlockEntityType {
@@ -42,7 +43,9 @@ public class BlockEntityTypeMixin<T extends BlockEntity> implements FabricBlockE
 
 	@Inject(method = "<init>", at = @At("RETURN"))
 	private void mutableBlocks(BlockEntityType.BlockEntitySupplier<? extends T> factory, Set<Block> blocks, Type<?> type, CallbackInfo ci) {
-		this.validBlocks = new HashSet<>(this.validBlocks);
+		if (this.validBlocks instanceof ImmutableSet<Block>) {
+			this.validBlocks = new HashSet<>(this.validBlocks);
+		}
 	}
 
 	@Override
diff --git a/ffapi.gradle.properties b/ffapi.gradle.properties
index c7e716b24..2b9f240c7 100644
--- a/ffapi.gradle.properties
+++ b/ffapi.gradle.properties
@@ -1,7 +1,7 @@
 loom.platform=neoforge
 fabric.loom.dontRemap=true
 
-implementationVersion=2.0.12
+implementationVersion=2.0.13
 
 versionMc=1.21.1
 versionForge=21.1.57