diff --git a/mrbgems/mruby-set/mrblib/set.rb b/mrbgems/mruby-set/mrblib/set.rb
index 91f2f852e..4858331a7 100644
--- a/mrbgems/mruby-set/mrblib/set.rb
+++ b/mrbgems/mruby-set/mrblib/set.rb
@@ -26,14 +26,10 @@ class Set
   # @param [Enumerable] enum The enumerable object to merge elements from
   # @return [Set] self
   def merge(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set merge
-      __merge(enum)
-    else
-      # General path: Add each element from the enumerable
-      __do_with_enum(enum) { add(_1) }
-      self
+    unless __merge(enum)
+      __do_with_enum(enum) { |o| add(o) }
     end
+    self
   end
 
   # Replaces the contents of the set with the contents of the given enumerable
@@ -52,14 +48,10 @@ class Set
   # @param [Enumerable] enum The enumerable object containing elements to remove
   # @return [Set] self
   def subtract(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set subtraction
-      __subtract(enum)
-    else
-      # General path: Remove each element from the enumerable
-      __do_with_enum(enum) { delete(_1) }
-      self
+    unless __subtract(enum)
+      __do_with_enum(enum) { |o| delete(o) }
     end
+    self
   end
 
   # Returns a new set containing elements common to the set and the given
@@ -68,15 +60,12 @@ class Set
   # @param [Enumerable] enum The enumerable object to find common elements with
   # @return [Set] A new set containing elements common to both
   def intersection(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set intersection
-      __intersection(enum)
-    else
-      # General path: Implement in Ruby for any enumerable
-      n = Set.new
-      __do_with_enum(enum) { n.add(_1) if include?(_1) }
-      n
-    end
+    n = __intersection(enum)
+    return n if n
+
+    n = Set.new
+    __do_with_enum(enum) { |o| n.add(o) if include?(o) }
+    n
   end
 
   # Alias for #intersection
@@ -88,13 +77,10 @@ class Set
   # @param [Enumerable] enum The enumerable object to merge with
   # @return [Set] A new set containing all elements from both
   def union(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set union
-      __union(enum)
-    else
-      # General path: Create a duplicate and merge the enumerable
-      dup.merge(enum)
-    end
+    n = __union(enum)
+    return n if n
+
+    dup.merge(enum)
   end
 
   # Aliases for #union
@@ -107,15 +93,12 @@ class Set
   # @param [Enumerable] enum The enumerable object to find elements to remove
   # @return [Set] A new set with elements from self that are not in enum
   def difference(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set difference
-      __difference(enum)
-    else
-      # General path: Create a duplicate and remove the enumerable elements
-      result = dup
-      __do_with_enum(enum) { result.delete(_1) }
-      result
-    end
+    n = __difference(enum)
+    return n if n
+
+    result = dup
+    __do_with_enum(enum) { |o| result.delete(o) }
+    result
   end
 
   # Alias for #difference
@@ -127,14 +110,11 @@ class Set
   # @param [Enumerable] enum The enumerable object to find exclusive elements with
   # @return [Set] A new set containing elements exclusive between both
   def ^(enum)
-    if enum.is_a?(Set)
-      # Fast path: Call C-implemented function for Set-to-Set XOR
-      __xor(enum)
-    else
-      # General path: Convert enum to a set and calculate (self|s2)-(self&s2)
-      s2 = Set.new(enum)
-      (self | s2) - (self & s2)
-    end
+    n = __xor(enum)
+    return n if n
+
+    s2 = Set.new(enum)
+    (self | s2) - (self & s2)
   end
 
   # Iterates over each element in the set.
diff --git a/mrbgems/mruby-set/src/set.c b/mrbgems/mruby-set/src/set.c
index b77be099c..d9556f2cb 100644
--- a/mrbgems/mruby-set/src/set.c
+++ b/mrbgems/mruby-set/src/set.c
@@ -672,6 +672,11 @@ static mrb_value
 set_core_merge(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
+
+  if (!set_is_set(other)) {
+    return mrb_false_value();
+  }
+
   kset_t *self_set = set_get_kset(mrb, self);
   kset_t *other_set = set_get_kset(mrb, other);
 
@@ -680,7 +685,7 @@ set_core_merge(mrb_state *mrb, mrb_value self)
     kset_copy_elements(mrb, self_set, other_set);
   }
 
-  return self;
+  return mrb_true_value();
 }
 
 /*
@@ -692,11 +697,15 @@ set_core_subtract(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
 
+  if (!set_is_set(other)) {
+    return mrb_false_value();
+  }
+
   kset_t *self_set = set_get_kset(mrb, self);
-  if (kset_is_empty(self_set)) return self;
+  if (kset_is_empty(self_set)) return mrb_true_value();
 
   kset_t *other_set = set_get_kset(mrb, other);
-  if (kset_is_empty(other_set)) return self;
+  if (kset_is_empty(other_set)) return mrb_true_value();
 
   /* Remove all elements that are in other set */
   KSET_FOREACH(other_set, k) {
@@ -707,7 +716,7 @@ set_core_subtract(mrb_state *mrb, mrb_value self)
     }
   }
 
-  return self;
+  return mrb_true_value();
 }
 
 /*
@@ -719,6 +728,10 @@ set_core_union(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
 
+  if (!set_is_set(other)) {
+    return mrb_nil_value();
+  }
+
   /* Create a new set by duplicating self */
   mrb_value result = mrb_obj_dup(mrb, self);
   kset_t *result_set = set_get_kset(mrb, result);
@@ -745,6 +758,10 @@ set_core_difference(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
 
+  if (!set_is_set(other)) {
+    return mrb_nil_value();
+  }
+
   /* Create a new set by duplicating self */
   mrb_value result = mrb_obj_dup(mrb, self);
   kset_t *result_set = set_get_kset(mrb, result);
@@ -778,6 +795,10 @@ set_core_intersection(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
 
+  if (!set_is_set(other)) {
+    return mrb_nil_value();
+  }
+
   /* Create a new empty set of the same class as self */
   mrb_value result = mrb_obj_new(mrb, mrb_obj_class(mrb, self), 0, NULL);
   kset_t *result_set = set_get_kset(mrb, result);
@@ -810,6 +831,11 @@ static mrb_value
 set_core_xor(mrb_state *mrb, mrb_value self)
 {
   mrb_value other = mrb_get_arg1(mrb);
+
+  if (!set_is_set(other)) {
+    return mrb_nil_value();
+  }
+
   mrb_value result = mrb_obj_new(mrb, mrb_obj_class(mrb, self), 0, NULL);
   kset_t *result_set = set_get_kset(mrb, result);
   kset_t *self_set, *other_set;