diff --git a/src/mdevaluate/correlation.py b/src/mdevaluate/correlation.py
index d78c43a..786f57b 100644
--- a/src/mdevaluate/correlation.py
+++ b/src/mdevaluate/correlation.py
@@ -100,16 +100,25 @@ def shifted_correlation(
         else:
             index = selector(frames[start_frame])
             if len(index) == 0:
-                return np.zeros(len(idx))
+                return np.zeros(len(shifted_idx))
 
-            if isinstance(index[0], int) or isinstance(index[0], bool):
+            elif isinstance(index[0], int) or isinstance(index[0], bool):
                 return get_correlation(frames, start_frame, index, shifted_idx)
             else:
                 correlations = []
                 for ind in index:
-                    correlations.append(
-                        get_correlation(frames, start_frame, ind, shifted_idx)
-                    )
+                    if len(ind) == 0:
+                        correlations.append(np.zeros(len(shifted_idx)))
+
+                    elif isinstance(ind[0], int) or isinstance(ind[0], bool):
+                        correlations.append(
+                            get_correlation(frames, start_frame, ind, shifted_idx)
+                        )
+                    else:
+                        raise ValueError(
+                            "selector has more than two dimensions or does not "
+                            "contain int or bool types"
+                        )
                 return correlations
 
     if 1 - skip < window: