diff --git a/river/sketch/hierarchical_heavy_hitters.py b/river/sketch/hierarchical_heavy_hitters.py
index 4c936f78c2..fa32f7daff 100644
--- a/river/sketch/hierarchical_heavy_hitters.py
+++ b/river/sketch/hierarchical_heavy_hitters.py
@@ -189,7 +189,7 @@ def compress(self):
         
     def _compress_node(self, node: HierarchicalHeavyHitters.Node):
         """Recursively compress nodes in the hierarchical tree."""
-        if not node.children:
+        if node is not None and not node.children:
             return
 
         for child_key, child_node in list(node.children.items()):
@@ -247,26 +247,31 @@ def __getitem__(self, key: typing.Hashable) -> int:
         """Get the count of a specific hierarchical key."""
         current = self.root
 
-        for i in range(len(key)):
-                
+        if isinstance(key, str):
+            for i in range(len(key)):
                 sub_key = key[:i + 1]
 
                 if sub_key not in current.children:
-
                     return 0
-                
+
                 current = current.children[sub_key]
 
                 if sub_key == key:
-               
-                   return current.ge
-                
+                    return current.ge
+        else:
+   
+            return 0
+
         return 0
  
             
     def totals(self) -> int:
         """Return the total number of elements in the hierarchical tree."""
-        return self._count_entries(self.root) -1
+        if self.root is not None:
+            total = self._count_entries(self.root) - 1
+        else:
+            total = 0
+        return total
     
     def _count_entries(self, node: HierarchicalHeavyHitters.Node) -> int:
         """Recursively count the total number of nodes in the hierarchical tree."""
diff --git a/river/sketch/hyper_log_log.py b/river/sketch/hyper_log_log.py
index 0ec28365ae..18281aad1b 100644
--- a/river/sketch/hyper_log_log.py
+++ b/river/sketch/hyper_log_log.py
@@ -114,7 +114,7 @@ def update(self, x: typing.Hashable):
         j = hash_val & (self.m - 1)
         w = hash_val >> self.b
         self.registers[j] = max(self.registers[j], self.left_most_one(w))
-        return
+        return None
        
     def count(self) -> int:
         """
diff --git a/river/sketch/space_saving.py b/river/sketch/space_saving.py
index 88de70cef0..5afef82c1d 100644
--- a/river/sketch/space_saving.py
+++ b/river/sketch/space_saving.py
@@ -72,13 +72,12 @@ def __init__(self, k: int):
 
     def update(self, x: typing.Hashable, w: int = 1):
         """Update the counts with the given element."""
+        
         if x in self.counts:
             self.counts[x] += w
-
         elif len(self.counts) >= self.k:
-            min_count_key = min(self.counts, key=self.counts.get)
+            min_count_key = min(self.counts, key=lambda k: self.counts[k])  # Use lambda to specify key function
             self.counts[x] = self.counts.pop(min_count_key, 0) + 1
-
         else:
             self.counts[x] = w