diff --git a/ext/concurrent-ruby-ext/atomic_fixnum.c b/ext/concurrent-ruby-ext/atomic_fixnum.c
index 088874006..2abe9c5f7 100644
--- a/ext/concurrent-ruby-ext/atomic_fixnum.c
+++ b/ext/concurrent-ruby-ext/atomic_fixnum.c
@@ -73,6 +73,7 @@ VALUE method_atomic_fixnum_update(VALUE self) {
   for (;;) {
     old_value = method_atomic_fixnum_value(self);
     new_value = rb_yield(old_value);
+    Check_Type(new_value, T_FIXNUM);
     if (ir_compare_and_set(self, old_value, new_value) == Qtrue) {
       return new_value;
     }
diff --git a/lib/concurrent-ruby/concurrent/atomic/mutex_atomic_fixnum.rb b/lib/concurrent-ruby/concurrent/atomic/mutex_atomic_fixnum.rb
index 0ca395579..132616b77 100644
--- a/lib/concurrent-ruby/concurrent/atomic/mutex_atomic_fixnum.rb
+++ b/lib/concurrent-ruby/concurrent/atomic/mutex_atomic_fixnum.rb
@@ -55,7 +55,7 @@ def compare_and_set(expect, update)
     # @!macro atomic_fixnum_method_update
     def update
       synchronize do
-        @value = yield @value
+        ns_set(yield @value)
       end
     end
 
diff --git a/spec/concurrent/atomic/atomic_fixnum_spec.rb b/spec/concurrent/atomic/atomic_fixnum_spec.rb
index 8222bf523..81b8e8bf4 100644
--- a/spec/concurrent/atomic/atomic_fixnum_spec.rb
+++ b/spec/concurrent/atomic/atomic_fixnum_spec.rb
@@ -152,6 +152,17 @@
       atomic = described_class.new(1000)
       expect(atomic.update { |v| v + 1 }).to eq 1001
     end
+
+    it 'rejects a non-integer result without changing the value' do
+      atomic = described_class.new(1000)
+
+      expect {
+        atomic.update { 'not an integer' }
+      }.to(raise_error { |error|
+        expect(error.class).to be(ArgumentError).or(be(TypeError))
+      })
+      expect(atomic.value).to eq 1000
+    end
   end
 end