diff --git a/Common/util_eddies.py b/Common/util_eddies.py
index c823856eb0714cbe7c910cdbde58d7d12a91ba76..5dc3a41e891b2b29f9db2fb026c19ff4f6268199 100644
--- a/Common/util_eddies.py
+++ b/Common/util_eddies.py
@@ -183,7 +183,11 @@ class SHPC_class:
         return i_slice
 
     def comp_ishape(self, date, eddy_index, i_slice, orientation):
-        """Compute the location in the shapefiles."""
+        """Compute the location in the shapefiles. Returns None if ishape_last
+        was not found and could not be created. Crashes if date is not
+        in i_slice. Crashes if eddy_index is out of range.
+
+        """
 
         ishape_last = self.get_ishape_last(i_slice, orientation)
 
@@ -203,6 +207,11 @@ class SHPC_class:
         return ishape
 
     def ishape_range(self, date, i_slice, orientation):
+        """Returns an empty list if ishape_last was not found and could not be
+        created. Crashes if date is not in i_slice.
+
+        """
+
         ishape_start = self.comp_ishape(date, 1, i_slice, orientation)
 
         if ishape_start is None: