diff --git a/src/main/java/io/github/skippyall/minions/gui/instruction/ConfigureInstructionGui.java b/src/main/java/io/github/skippyall/minions/gui/instruction/ConfigureInstructionGui.java
index 74ca975..d8e0834 100644
--- a/src/main/java/io/github/skippyall/minions/gui/instruction/ConfigureInstructionGui.java
+++ b/src/main/java/io/github/skippyall/minions/gui/instruction/ConfigureInstructionGui.java
@@ -124,16 +124,26 @@ public class ConfigureInstructionGui extends MinionsGui implements ConfiguredIns
     }
 
     private void updateRunSlot() {
-        if(!instruction.isRunning()) {
-            gui.setSlot(26, new GuiElementBuilder(Items.ARROW)
-                    .setName(Component.translatable("minions.gui.instruction.run"))
-                    .setCallback(() -> instruction.run(minion.getInstructionManager()))
-            );
+        List<Component> errors = instruction.preCheck();
+        if(errors.isEmpty()) {
+            if (!instruction.isRunning()) {
+                gui.setSlot(26, new GuiElementBuilder(Items.ARROW)
+                        .setName(Component.translatable("minions.gui.instruction.run"))
+                        .setCallback(() -> instruction.run(minion.getInstructionManager()))
+                );
+            } else {
+                gui.setSlot(26, new GuiElementBuilder(Items.BARRIER)
+                        .setName(Component.translatable("minions.gui.instruction.stop"))
+                        .setCallback(() -> instruction.stop(minion.getInstructionManager()))
+                );
+            }
         } else {
-            gui.setSlot(26, new GuiElementBuilder(Items.BARRIER)
-                    .setName(Component.translatable("minions.gui.instruction.stop"))
-                    .setCallback(() -> instruction.stop(minion.getInstructionManager()))
-            );
+            GuiElementBuilder builder = new GuiElementBuilder(Items.RED_WOOL)
+                    .setName(Component.translatable("minions.gui.instruction.errors"));
+            for(Component error : errors) {
+                builder.addLoreLine(error);
+            }
+            gui.setSlot(26, builder);
         }
     }
 
diff --git a/src/main/java/io/github/skippyall/minions/minion/fakeplayer/MinionFakePlayer.java b/src/main/java/io/github/skippyall/minions/minion/fakeplayer/MinionFakePlayer.java
index ed3279a..be08b24 100644
--- a/src/main/java/io/github/skippyall/minions/minion/fakeplayer/MinionFakePlayer.java
+++ b/src/main/java/io/github/skippyall/minions/minion/fakeplayer/MinionFakePlayer.java
@@ -51,12 +51,14 @@ import net.minecraft.world.level.storage.ValueInput;
 import net.minecraft.world.level.storage.ValueOutput;
 import net.minecraft.world.phys.Vec2;
 import net.minecraft.world.phys.Vec3;
+import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Consumer;
 
+@NullMarked
 public class MinionFakePlayer extends ServerPlayer {
     public Runnable fixStartingPosition = () -> {};
 
@@ -310,6 +312,7 @@ public class MinionFakePlayer extends ServerPlayer {
     public void dropAllDeathLoot(ServerLevel world, DamageSource damageSource) {
         super.dropAllDeathLoot(world, damageSource);
         ItemEntity entity = drop(toItemStack(world.getServer()), true, false);
+        //noinspection ConstantValue (Wrong nullability of drop)
         if (entity != null) {
             entity.setUnlimitedLifetime();
         }
diff --git a/src/main/java/io/github/skippyall/minions/program/conversion/ConverterList.java b/src/main/java/io/github/skippyall/minions/program/conversion/ConverterList.java
index 6222e48..54fac0a 100644
--- a/src/main/java/io/github/skippyall/minions/program/conversion/ConverterList.java
+++ b/src/main/java/io/github/skippyall/minions/program/conversion/ConverterList.java
@@ -13,6 +13,7 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.ListIterator;
 import java.util.Objects;
+import java.util.function.Consumer;
 
 public class ConverterList {
     public static final Codec<ConverterList> CODEC = ValueConverter.CODEC.listOf().xmap(ConverterList::new, l -> l.converters);
@@ -55,7 +56,11 @@ public class ConverterList {
     private <F,I,T> Result<TypedValue<?>, Component> convert(TypedValue<F> from, ValueConverter<I,T> converter, ListIterator<ValueConverter<?,?>> iterator) {
         Result<I, Component> inter = Casts.castOrError(from, converter.getFrom());
         if(inter instanceof Result.Error<I, Component> error) {
-            return new Result.Error<>(Component.translatable("minions.converter.list.passing_error", iterator.previousIndex(), error.message()));
+            return new Result.Error<>(
+                    Component.translatable("minions.converter.list.passing_error", iterator.previousIndex())
+                            .append("\n")
+                            .append(error.message())
+            );
         }
         Result<T, Component> to = converter.convert(inter.getOrThrow());
 
@@ -81,6 +86,24 @@ public class ConverterList {
         return warning;
     }
 
+    public void check(Consumer<Component> errorConsumer, ValueType<?> input, ValueType<?> output) {
+        Component firstCastWarning = createCastWarning(input, converters.isEmpty() ? output : converters.get(0).getFrom());
+        if(firstCastWarning != null) {
+            errorConsumer.accept(firstCastWarning);
+        }
+        for(int i = 0; i < converters.size(); i++) {
+            ValueConverter<?,?> converter = converters.get(i);
+            Component converterWarning = createConverterWarning(converter);
+            if(converterWarning != null) {
+                errorConsumer.accept(converterWarning);
+            }
+            Component castWarning = createCastWarning(converter.getTo(), i + 1 < converters.size() ? converters.get(i + 1).getFrom() : output);
+            if(castWarning != null) {
+                errorConsumer.accept(castWarning);
+            }
+        }
+    }
+
     @Override
     public boolean equals(Object o) {
         if (!(o instanceof ConverterList that)) return false;
diff --git a/src/main/java/io/github/skippyall/minions/program/supplier/ValueSupplierList.java b/src/main/java/io/github/skippyall/minions/program/supplier/ValueSupplierList.java
index 97834f9..4faa2c5 100644
--- a/src/main/java/io/github/skippyall/minions/program/supplier/ValueSupplierList.java
+++ b/src/main/java/io/github/skippyall/minions/program/supplier/ValueSupplierList.java
@@ -158,9 +158,8 @@ public class ValueSupplierList<R extends InstructionRuntime<R>> {
             return convertedResult.flatMap(convertedValue -> Casts.castOrError(convertedValue, parameter.type()));
         }
 
-        public @Nullable Component check(Consumer<Component> errorConsumer) {
-            //TODO check it
-            return null;
+        public void check(Consumer<Component> errorConsumer) {
+            converters.check(errorConsumer, parameter.type(), supplier.getValueType());
         }
 
         public static <R extends InstructionRuntime<R>> MapCodec<ValueSupplierEntry<?,R>> getCodec(Codec<ValueSupplier<?,R>> argumentCodec) {
diff --git a/src/main/resources/assets/minions/lang/en_us.json b/src/main/resources/assets/minions/lang/en_us.json
index 70d534d..eabe971 100644
--- a/src/main/resources/assets/minions/lang/en_us.json
+++ b/src/main/resources/assets/minions/lang/en_us.json
@@ -34,6 +34,7 @@
   "minions.gui.instruction.configure.delete.confirm": "Delete %s?",
   "minions.gui.instruction.configure.copy": "Copy Reference",
   "minions.gui.instruction.configure.copy.description": "Click here and then use a Minion Trigger Block to bind it",
+  "minions.gui.instruction.errors": "Errors",
   "minions.gui.instruction.last_errors": "Last Errors",
   "minions.gui.instruction.run": "Run",
   "minions.gui.instruction.stop": "Stop",