diff --git a/src/esbmtk/extended_classes.py b/src/esbmtk/extended_classes.py
index dd6db7f7..22af17a4 100644
--- a/src/esbmtk/extended_classes.py
+++ b/src/esbmtk/extended_classes.py
@@ -317,11 +317,12 @@ def __init__(self, **kwargs) -> None:
                 raise SourceSinkPropertiesError(
                     f"{s.n} needs to be a valid species name"
                 )
-            if s in self.isotopes:
-                isotopes = self.isotopes[s]
+            if isinstance(self.isotopes, dict):
+                if s in self.isotopes:
+                    isotopes = self.isotopes[s]
             else:
-                isotopes = False
-
+                isotopes = self.isotopes
+                    
             if type(self).__name__ == "SourceProperties":
                 a = Source(
                     name=f"{s.name}",
diff --git a/src/esbmtk/sealevel.py b/src/esbmtk/sealevel.py
index 2cbc83be..b9320738 100644
--- a/src/esbmtk/sealevel.py
+++ b/src/esbmtk/sealevel.py
@@ -148,7 +148,12 @@ def read_data(self) -> None:
         max_el = df.iloc[0, 0] - self.max_elevation
         elevation = df.iloc[max_el:, 0].to_numpy()
         area = df.iloc[max_el:, 1].to_numpy() * self.sa
-        
+
+        dz = df.iloc[0, 0] - df.iloc[1, 0]
+        if dz != 1:  # in case we need to interpolate the data
+            dzi = np.arange(elevation[0], elevation[-1], -1)
+            area = np.interp(dzi, elevation, area)
+            elevation = dzi
         # create lookup table with area and area_dz
         self.hypdata = np.column_stack(
             (
@@ -197,7 +202,7 @@ def area(self, elevation: int) -> float:
                 )
             )
 
-        return self.hypdata[i,1]
+        return self.hypdata[i, 1]
 
     def area_dz(self, u: float, l: float) -> float:
         """calculate the area between two elevation datums
@@ -230,7 +235,7 @@ def area_dz(self, u: float, l: float) -> float:
 
         u = self.max_elevation - int(u)
         l = self.max_elevation - int(l)
-        
+
         return self.hypdata[u, 1] - self.hypdata[l, 1]
 
     def volume(self, u: float, l: float) -> float:
@@ -430,8 +435,7 @@ def slice_count(
     """
     sub_grid = grid[start:end, ...]
 
-    count = np.zeros(int((elevation_maximum - elevation_minimum) // dz),
-                     dtype=float)
+    count = np.zeros(int((elevation_maximum - elevation_minimum) // dz), dtype=float)
 
     for i, e in enumerate(elevations):
         a = np.sum(np.logical_and(sub_grid > e, sub_grid < e + dz), axis=1)
@@ -480,5 +484,5 @@ def process_slice(
     lat_slice = lat[start:end]
     weight = np.array([grid_area(lat_val, dx) for lat_val in lat_slice])
     return slice_count(
-        start, end, weight, grid, elevation_minimum, elevation_maximum,
-        elevations, dz)
+        start, end, weight, grid, elevation_minimum, elevation_maximum, elevations, dz
+    )