diff --git a/src/main/java/io/bioimage/modelrunner/model/processing/Processing.java b/src/main/java/io/bioimage/modelrunner/model/processing/Processing.java
index fb3020f6..5c78f454 100644
--- a/src/main/java/io/bioimage/modelrunner/model/processing/Processing.java
+++ b/src/main/java/io/bioimage/modelrunner/model/processing/Processing.java
@@ -34,6 +34,7 @@
 import io.bioimage.modelrunner.tensor.Tensor;
 import net.imglib2.type.NativeType;
 import net.imglib2.type.numeric.RealType;
+import net.imglib2.util.Cast;
 
 /**
  * Class that executes the pre- or post-processing associated to a given tensor.
@@ -132,15 +133,18 @@ List<Tensor<R>> preprocess(List<Tensor<T>> tensorList){
      */
 	public <T extends RealType<T> & NativeType<T>, R extends RealType<R> & NativeType<R>>
 	List<Tensor<R>> preprocess(List<Tensor<T>> tensorList, boolean inplace) {
+		List<Tensor<R>> outputs = new ArrayList<Tensor<R>>();
+		if (preMap.entrySet().size() == 0) return Cast.unchecked(tensorList);
 		for (Entry<String, List<TransformationInstance>> ee : this.preMap.entrySet()) {
 			Tensor<T> tt = tensorList.stream().filter(t -> t.getName().equals(ee.getKey())).findFirst().orElse(null);
 			if (tt == null)
 				continue;
-			ee.getValue().forEach(trans -> {
-					trans.run(tt, inplace);
-			});
+			for (TransformationInstance trans : ee.getValue()) {
+				List<Tensor<R>> outList = trans.run(tt, inplace);
+				outputs.addAll(outList);
+			}
 		}
-		return null;
+		return outputs;
 	}
     
     /**
@@ -174,14 +178,17 @@ List<Tensor<R>> postprocess(List<Tensor<T>> tensorList){
      */
 	public <T extends RealType<T> & NativeType<T>, R extends RealType<R> & NativeType<R>>
 	List<Tensor<R>> postprocess(List<Tensor<T>> tensorList, boolean inplace) {
+		List<Tensor<R>> outputs = new ArrayList<Tensor<R>>();
+		if (postMap.entrySet().size() == 0) return Cast.unchecked(tensorList);
 		for (Entry<String, List<TransformationInstance>> ee : this.postMap.entrySet()) {
 			Tensor<T> tt = tensorList.stream().filter(t -> t.getName().equals(ee.getKey())).findFirst().orElse(null);
 			if (tt == null)
 				continue;
-			ee.getValue().forEach(trans -> {
-					trans.run(tt, inplace);
-			});
+			for (TransformationInstance trans : ee.getValue()) {
+				List<Tensor<R>> outList = trans.run(tt, inplace);
+				outputs.addAll(outList);
+			}
 		}
-		return null;
+		return outputs;
 	}
 }