diff --git a/src/preprocessing/categorical_encoder.rs b/src/preprocessing/categorical_encoder.rs
index 706670b..7e71119 100644
--- a/src/preprocessing/categorical_encoder.rs
+++ b/src/preprocessing/categorical_encoder.rs
@@ -156,7 +156,7 @@ impl OneHotEncoder {
     }
 
     /// Transform categorical variables to one-hot encoded and return a new matrix
-    pub fn transform<T: Categorizable, M: Matrix<T>>(&self, x: &M) -> Option<M> {
+    pub fn transform<T: Categorizable, M: Matrix<T>>(&self, x: &M) -> Result<M, Failed> {
         let (nrows, p) = x.shape();
         let additional_params: Vec<usize> = self
             .series_encoders
@@ -164,28 +164,24 @@ impl OneHotEncoder {
             .map(|enc| enc.num_categories)
             .collect();
 
-        let new_param_num: usize = p + additional_params.iter().fold(0, |cs, &v| cs + v - 1);
+        // Eac category of size v adds v-1 params
+        let expandws_p: usize = p + additional_params.iter().fold(0, |cs, &v| cs + v - 1);
+
         let new_col_idx = find_new_idxs(p, &additional_params[..], &self.col_idx_categorical[..]);
-        let mut res = M::zeros(nrows, new_param_num);
-        // copy old data in x to their new location
-        for (old_p, &new_p) in new_col_idx.iter().enumerate() {
-            for r in 0..nrows {
-                let val = x.get(r, old_p);
-                res.set(r, new_p, val);
-            }
-        }
+        let mut res = M::zeros(nrows, expandws_p);
+
         for (pidx, &old_cidx) in self.col_idx_categorical.iter().enumerate() {
             let cidx = new_col_idx[old_cidx];
-            let col_iter = (0..nrows).map(|r| res.get(r, cidx).to_category());
+            let col_iter = (0..nrows).map(|r| x.get(r, old_cidx).to_category());
             let sencoder = &self.series_encoders[pidx];
             let oh_series: Vec<Option<Vec<T>>> = sencoder.transform_iter(col_iter);
 
             for (row, oh_vec) in oh_series.iter().enumerate() {
                 match oh_vec {
                     None => {
-                        // Bad value in a series causes in to be invalid
-                        // todo: proper error handling, so user can know where the bad value is
-                        return None;
+                        // Since we support T types, bad value in a series causes in to be invalid
+                        let msg = format!("At least one value in column {} doesn't conform to category definition", old_cidx);
+                        return Err(Failed::transform(&msg[..]));
                     }
                     Some(v) => {
                         // copy one hot vectors to their place in the data matrix;
@@ -196,7 +192,27 @@ impl OneHotEncoder {
                 }
             }
         }
-        Some(res)
+
+        // copy old data in x to their new location while skipping catergorical vars (already treated)
+        let mut skip_idx_iter = self.col_idx_categorical.iter();
+        let mut cur_skip = skip_idx_iter.next();
+
+        for (old_p, &new_p) in new_col_idx.iter().enumerate() {
+            // if found treated varible, skip it
+            if let Some(&v) = cur_skip {
+                if v == old_p {
+                    cur_skip = skip_idx_iter.next();
+                    continue;
+                }
+            }
+
+            for r in 0..nrows {
+                let val = x.get(r, old_p);
+                res.set(r, new_p, val);
+            }
+        }
+
+        Ok(res)
     }
 }